Manchester United need new defenders, especially after their start to the new season.

Of course, that statement isn't helped by the Red Devils' convincing 3-0 win over Newcastle United last Sunday—in which the defence was rarely troubled—but the fact remains United could do with strengthening across their back-line.

In central defence, Rio Ferdinand is struggling to keep up with the pace in the Premier League, while Jonny Evans is too inconsistent to be a regular starter for the team, and even Nemanja Vidic is currently having difficulties following his return from injury.

While at left-back, Patrice Evra, Newcastle game aside, has continued his poor form, and on the other wing, Rafael has long been the only available right-back with Phil Jones and Chris Smalling racking up an ever-increasing injury history list.

That has led United to concede more shots and more goals than any of their title rivals and has been the reason for most of their poor results this campaign.

So in the upcoming January transfer window, it's clear Manchester United could do with bolstering their defence.

With that in mind, here are some world-class defenders—or those with world-class potential—which the Old Trafford outfit should realistically target.

Andrea Ranocchia

Inter Milan centre-back Andrea Ranocchia, 24, has unassailably been the Nerazzuri's best player this season and is an ideal replacement for the aging Rio Ferdinand.

Ranocchia has started every game for his team this season and has immediately proven his worth with a number of fine performances, most recently in the 1-0 derby win over AC Milan.

His tackling, intercepting and willingness to put his body on the line in that match were crucial to Inter gaining a vital three points, and even more importantly, the city bragging rights.

Those three qualities displayed by Ranocchia have been lacking in the Manchester United back-line for much of the past two seasons, and for a player schooled in the fine art of catenaccio, the Italian has what it takes to become a success both in the Premier League and—thanks to his stellar technical ability—at Old Trafford.

According to TuttoSport (link in Italian), the Red Devils could try to sign Ranocchia this January, having sent scouts to watch him in action against Torino back in September.

Valued at £12 million by TransferMarkt.co.uk, if United were able to get him for around that price, then he's a must-target player in the next transfer window.

Rafael Toloi

A Football Manager favourite, 21-year-old Brazilian centre-back Rafael Toloi is a player who has long been on the radar of Manchester United fans on Internet forums, and most probably the club itself.

In the summer of 2011, Manchester City were linked with the player, and there's no doubt he has what it takes to make it in the Premier League.

Now in late 2012, having switched to South American giants Sao Paulo and become a regular in their team, it could soon be time for Toloi to make that big move to Europe.

And for United, Toloi could be a perfect signing for a number of reasons.

As a defender, he has all the raw attributes—tackling, intercepting, clearing, shot-blocking and strength—to be a success in the English top flight, while he also has a great understanding of attacking movements to cope with the pace of even the quickest forwards.

But seeing as there are a lot of talents out there with such raw ability, what makes Toloi ideal is both his experience at a young age and his injury history.

For the past three seasons, Toloi has been a regular first-team starter and has only really missed matches through the occasional suspension.

Given United's frailties in defence thanks to injuries, having a reliable option in the squad is very much needed right now.

Discipline and aerial ability are two things preventing Toloi from already being world-class.

But there's no doubt he has world-class potential, and he'd only get better and better under the tutelage of Sir Alex Ferguson.

Filipe Luis

Filipe Luis is a player Manchester United could do with right now.

In a team desperate for consistency on the left wing, the Atletico Madrid left-back, 27, would make an ideal signing.

Having undoubtedly been Atletico's second-best player last season behind Radamel Falcao, Luis provides exactly what United are missing at left-back this season.

For the past two seasons in La Liga, Luis has been a shining example of how to be positionally aware, track back when necessary and, most importantly, knowing when to make a tackle.

Those are qualities Patrice Evra has lacked in the previous two campaigns, and while Alexander Buttner can rectify a few of those issues when he gets the chance, having an option who's experienced and more reliable than Evra is really what the team seems to need right now.

The Brazilian defender has signed a new five-year contract with Los Colchoneros back in the summer, but as far as Manchester United are concerned, he's definitely a player worth targeting.

Diego Reyes

Diego Reyes is quite possibly the reason why Mexico won the Olympic gold medal in the men's football at London 2012.

The 20-year-old centre-back, especially in his ability to keep out Neymar and Co. in the Olympic final against Brazil, has proven himself as one of the best young defenders in world football.

And his ability and style immediately scream of two clubs—Manchester United and Arsenal.

Thanks to the improved grassroots coaching and renovating of the antiquated football system in Mexico, the country is producing a ton of great footballers who are very technically gifted.

In terms of defenders, Reyes is among the best of them, being an extremely comfortable player on the ball who has the know-how and ability to play his way out of trouble without resorting to ineffective clearances which concede possession.

That combined with his English-style approach to defending, being robust but highly accurate in the tackle and aerially makes him perfect for the Premier League and in particular United or the Gunners.

And considering he's another player with very good fitness levels and no significant injury history, it'd certainly be in United's interest to snap him up this January.

Brede Hangeland

With uncertainty clouding his Fulham future, now could be the time to pounce for Manchester United.

Brede Hangeland's contract at Craven Cottage runs out in June 2013, and with no new deal on the cards, the Red Devils could pick up the experienced Premier League defender for a bargain fee this January.

He may be 31 years old, but Hangeland would be an ideal remedy to United's current defensive problems.

As far as his qualities go, the big Norwegian would give United something only Nemanja Vidic currently provides—domineering aerial ability.

In fact, this season Hangeland is doing it even better than Vidic, winning three aerial duels per game to Vidic's 2.5 while he also reads the play better than any United defender, having made more interceptions per game (h/t WhoScored.com).

And what also makes him a perfect fit for the Red Devils is his ability to play the ball out from the back—Hangeland has an 87.4 percent pass success rate from an average of 47 passes per game—as well as his injury history.

Hangeland has virtually a clean bill of health with no known ongoing muscular or joint problems, and that allowed him to play in every single Premier League game for Fulham last season and miss just one game the season before.

An experienced, reliable, physically fit defender who can improve United on the pitch, Hangeland is a must-target player for the club this January.

Jores Okore

Jores Okore is just 20 years old, but the FC Nordsjaelland centre-back can already be considered a world-class defender.

Having been his team's best player in the UEFA Champions League this season, Okore clearly has what it takes to make it at the top and will be highly sought after this summer.

Which is why it would be clever for United to make a move for him this January, with club legend Peter Schmeichel urging Sir Alex Ferguson to sign him up.

The young defender, like a lot of good young defensive talents, has the strength and aerial ability to make it in the Premier League, but what could potentially make him so successful is his exemplary reading of the game despite racking up less than 60 appearances as a professional as well as his unparalleled pace for a centre-back.

He proved against Chelsea he can defend against the best, and in a division such as the Premier League, where clubs are constantly working to make their players better athletes, and thus, the pace of the game will consistently increase, having a defender like Okore in the team will be a big asset.

Especially for United, who don't have any notably fast defenders, and considerably a slow one in Rio Ferdinand.

Back in February, the Sky Sports Talent Scout gave Jores Okore a current value of £3 million and a potential value of £15 million.

If Manchester United make a move in January for Okore, they could get the centre-back for a fee in the middle of the two aforementioned values.

And for a player of his ability and more importantly his potential, getting him in January for that price is a must-do deal.

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