Boxing trainer Freddie Roach believes Floyd Mayweather won't accept a fight with Amir Khan, because the British fighter's tool set will seriously trouble his unbeaten record.

Khan's speed is often used to counter opponents who possess greater natural power. The Bolton-based brawler doesn't flaunt a particularly strong jaw, but Roach believes Mayweather doesn't have the strength to put him down, as reported by Jeff Powell of the Mail Online:

It’s not going to happen (a fight between Mayweather and Khan). Floyd knows that Amir’s speed would trouble him. He is too careful with his choice of opponents to take that risk. Floyd also knows he himself doesn’t possess the knock-out punch which would make him more certain of winning that fight.

To further insult Money, Roach also revealed that Manny Pacquiao had consistently floored Khan (with extra-padded gloves) during their sparring sessions in his Los Angeles-based Wild Card gym. "Manny frequently put Amir on the floor, knocking him cold four or five times," said Roach, per Powell.

Khan has long craved an opportunity to take on Mayweather. Despite calling Khan a "solid, good competitor," Mayweather brushed off the 27-year-old's threat in September. 

"I mean I think when Amir Khan was an amateur, I was champion," Mayweather told the media after beating Marcos Maidana. "And then when Amir Khan turned pro and got knocked out, I was champion. Then when Amir Khan got knocked out again I was champion. And then when he lost his belt again I'm still champion and I'm still right here."

Khan dominated Luis Collazo with an impressive performance at the MGM Grand earlier in the year, but the three losses on his record provide Mayweather with a reason not to accept a fight. The American has been able to use this against Khan ever since speculation began to pair the two together, as Mayweather suggests the Englishman isn't worthy of standing in the ring with him.

Roach believes Pacquiao remains "the one really big threat" to Floyd's historic run of 47 fights unbeaten, as reported by Powell. "And that is why this is the only fight which can raise enough money to get Floyd into the ring with someone truly dangerous," said the trainer.

Khan can perhaps change that with a big win over Devon Alexander in December. The former Olympic silver medalist is pushing himself to receive the shot against Mayweather, who beat Maidana in his last two fights. A change of pace would certainly be interesting, particularly if Khan's challenge is accepted.

A bout against Pacquiao would undoubtedly blow that fight out of the water, however.

Mayweather and Pac-Man would have the opportunity to crown their careers with a victory over the other, something the former wouldn't receive if he decided to take on Khan. The risk would certainly be coming from Pretty Boy if he accepted a bout against Khan, who couldn't reasonably be expected to topple him.