Closing in on the halfway mark of the 2019 Major League Baseball season, there have been plenty of surprises on the field that have had a profound impact on fantasy teams. 

Few people would have predicted coming into the year that the Minnesota Twins would be leading MLB in OPS (.856), OPS+ (127), runs scored (382) and total bases (1,141) going into Tuesday night's game against the Seattle Mariners. 

Since we have a significant sample size by which it's possible to judge which players are trending upward and who looks like a disappointment, here are the key names to target on the waiver wire this week. 

Yandy Diaz, 1B/3B, Tampa Bay Rays

Since being acquired by the Rays from the Cleveland Indians as part of a three-team trade during the offseason, Yandy Diaz has turned into one of the key players for a club challenging the New York Yankees atop the American League East. 

The 27-year-old did miss two weeks from May 20-June 1 with an ankle injury, but he has since returned to the lineup without any apparent restriction. He hit .324/.405/.432 in his first nine games back. 

One reason for Diaz's improved performance in 2019 is because the Rays got him to tap into more power without significantly altering his swing mechanics:

Perhaps it's because the Rays don't generate a lot of attention from fans in their own city, let alone from national audiences, but Diaz only being owned in 33 percent of Yahoo and ESPN leagues is almost criminally low. 

The Cuban entered Tuesday with more homers (10) than Boston Red Sox third baseman Rafael Devers (nine) and ninth among all AL third baseman with 24 RBI. 

There is no reason for a player like Diaz, who should be a fantasy regular at this point, to still be available in 67 percent of all leagues. He's having a breakout season with the kind of plate discipline and power to keep up his performance the rest of the year. 

Roberto Perez, C, Cleveland Indians

The state of catching in MLB is dire. 

Per FanGraphs, there are only five catchers who have enough plate appearances to qualify for the batting title.

From that group, J.T. Realmuto of the Philadelphia Phillies, Yasmani Grandal of the Milwaukee Brewers and Willson Contreras of the Chicago Cubs are the only ones who have been worth more than one win above replacement. 

Trying to find an impact fantasy catcher has become an extremely difficult task, for many reasons. The standard for players at the position is tied to their defensive value, and anything they contribute on offense is almost a bonus. 

One pleasant surprise this season has been Roberto Perez of the Indians. He was given the starting job for the first time in his career after Yan Gomes was sent to the Washington Nationals during the offseason. 

The 30-year-old has already set a career high with 11 homers⁠—one more than his combined total from 2017-18—and has been an underrated offensive asset in 2019:

There are certainly limitations to Perez's upside in fantasy. His .232 average entering Tuesday was the best of his career in a season when he's had at least 100 plate appearances, but his strikeout rate of 26.7 percent, while high, is actually lower than New York Yankees star Gary Sanchez (27.6).

Perez is readily available in 99 percent of ESPN fantasy leagues and 86 percent of Yahoo leagues. Unless you have one of those top-tier catchers on your roster, there's no reason not to take a flier on a player who is going to get regular at-bats and to this point has taken advantage of an expanded role. 

Anibal Sanchez, SP, Washington Nationals

To this point, Anibal Sanchez's season can be divided into two parts. The first part runs from April 3-May 16 when he had a 5.10 ERA, 46 hits allowed and an .816 OPS against in nine starts. 

The Nationals placed the 35-year-old on the injured list May 17 with a sore hamstring after lasting just 1.1 innings against the New York Mets the previous day. 

In three starts since coming off the injured list, Sanchez has been terrific with a 1.04 ERA, 15 strikeouts and nine hits allowed over 17.1 innings.

Three starts is a small sample size, and two were against a Chicago White Sox team that ranks 21st in MLB in runs scored.

If you want to be really optimistic about Sanchez keeping up this pace for the remainder of the year, you can rightly point out he was also very good last year with a 2.83 ERA in 25 appearances for the Atlanta Braves. 

Sanchez's fantasy value is rising right now, even though he's not overpowering lineups and is being closely monitored by Washington's coaching staff. He hasn't thrown more than 89 pitches in a start since May 5. 

Fantasy owners have largely kept away from Sanchez to this point. He's only owned in 29 percent of Yahoo leagues and seven percent of ESPN leagues.  

If your roster currently has a starter on the injured list or lacks depth, Sanchez is one of the best available options who won't cost you anything except a roster spot. 

Fantasy availability via FantasyPros.com