Three key players to watch in the NL

Earlier, I looked at three key American League players. Here are three guys in the National League who could provide a big in-house improvement the rest of the way.

Scott Rolen, 3B, Reds

Rolen was a big key to the Reds' division title in 2010, batting .285/.358/.497 and providing a solid right-handed bat between Joey Votto and Jay Bruce. He's been terrible this season, with a .242/.279/.399 line and just five home runs in 243 at-bats, unacceptable numbers for the Great American Ballpark bandbox. Rolen does have 20 doubles, indicating some increased power potential is there. Dusty Baker has moved him down to sixth in the order. The Reds are unlikely to trade for a replacement given Rolen's veteran status, but he's showing signs of an aging player who has battled injuries through his career. The indicator: His walk rate is 3.8 percent, nearly six percent off last season and well below his career rate of 10.6 percent. Often, as a player ages and his bat speed declines, he has to guess more to "speed up" his bat; that leads to chasing more pitches or swinging earlier in the count. While the Reds' rotation has many issues, it's Rolen's bat that may provide the best hope for a stretch-drive kick.

Stephen Drew, SS, Diamondbacks

Drew has alternated good years with mediocre ones during his career. He hit .316 as a rookie and then .238 in his full season. In 2008, he hit .291 with 21 home runs; his OPS fell nearly 100 points in 2009. Last season, he hit .278 with a .352 on-base percentage. This season, he's hitting .254/.320/.370, an OPS nearly 100 points lower than 2010. Who is the real Stephen Drew? I think he has a good chance to improve his numbers in the second half, given that he's hitting just .223 with three home runs at home. In a ballpark conducive to hitting, those are terrible numbers. If Drew can produce an .800 OPS -- or higher -- the rest of the way, Arizona's lineup becomes one of the deepest in the league. Drew is hitting just .217 against lefties, and second baseman Kelly Johnson is hitting just .198, meaning the D-backs would also be wise to pick up a right-handed hitting middle-infielder to spell the lefty hitting Drew and Johnson. You can't worry about bruising egos at this point. (Of course, modern-day roster construction makes it difficult to carry too many platoon players, since every teams keeps seven or eight relievers.)

Colby Rasmus, CF, Cardinals

As expected, Kyle Lohse is starting to regress -- he's allowed four runs or more in five of his past seven starts -- meaning the burden of capturing the NL Central crown will continue to be placed on the Cardinals' offense. This was supposed to be the breakout season for the 24-year-old Rasmus, after he hit .276 with 23 home runs last season in 464 at-bats. While Tony La Russa no longer holds him back against some lefties, Rasmus' OPS has declined from .859 to .730, leading to trade rumor speculation. Rasmus has cut his strikeout rate from 2010, but it hasn't translated into bigger numbers. Surprisingly, he's hit lefties better than righties (.844 versus .692 OPS). With Jon Jay hitting .310 with a .360 OBP, moderate power and possessing the ability to play center, maybe the Cardinals will shop around Rasmus to upgrade the pitching staff. I wouldn't do it, but Rasmus could be one of the best trade chips on the market.