Ian Stewart has been an enigma. In 2004 Baseball America ranked Stewart the fourth best prospect in baseball behind Delmon Young, Felix Hernandez, and Joe Mauer. In 2011, Stewart managed a 13 wRC+ in 136 plate appearances; in other words, his offense was 87 percent below league average. While Stewart had a couple of mediocre seasons in 2008, 2009, and 2010, he has not lived up to his promise.
Stewart does, however, have the makings of a productive hitter. His walk rate (BB%) has been above league average in each of his past four seasons. His isolated power (ISO) has been above league average in three of his past four seasons, (his disastrous 2011 being the exception). In 2008 and 2010 his line drive percentage (LD%) was approximately above the 90th percentile among all players.* Despite these promising signs, Stewart's best offensive season in the majors resulted in a 100 wRC+ - exactly league average - and a 1.1 WAR, and his best overall season - fielding included - resulted in a 97 wRC+ and a 1.5 WAR.
People have pointed to Stewart's injuries as well as his lack of stable major league playing time as reasons for his struggles. Stewart should be able to put both of those concerns behind him in 2012 as he is currently healthy and penciled in as the Cubs' starting third baseman. While I am eager to see what Stewart can do with regular playing time, I'd like to examine another option that some have recently mentioned: a platoon between Stewart and Baker.
Here are Jeff Baker's splits:
Jeff Baker hits significantly better against lefties (119 wRC+) than he does against righties (63 wRC+). While Baker benefitted from a .388 BABIP against lefties in 2010, he also benefitted against righties in 2009 with a .371 BABIP.
Here are Ian Stewart's splits:
At first glance, it looks like Ian Stewart hits about the same against righties (91 wRC+) and against lefties (88 wRC+). However, Stewart greatly benefitted from a .444 BABIP against lefties in 2008, which consequently skewed his offensive numbers in that year. If we use 2009 and 2010 - both significantly larger sample sizes than 2008 and 2011 - to calculate Stewart's wRC+ against lefties, we arrive at an average wRC+ of 71. We can now see that Stewart hits righties (91 wRC+) better than he does lefties (71 wRC+).
Assuming that Stewart and Baker will be the only two players to play third base, that there will be 175 plate appearances against lefties and 450 plate appearances against righties available for the two of them combined, and that Stewart's average wRC+ is 71 instead of 88, here is how a platoon would help.*
Together, the two of them could provide league average offense at third base - something that neither of them are likely capable of doing on their own, (unless Stewart's 2011 was entirely a fluke). While a 99 wRC+ isn't ideal when you're trying to replace Ramirez's 133 wRC+, it's better than the options the Cubs had prior to the trade, and it comes at a pretty cheap price.
I'd like to reiterate that I, personally, would like to see what Stewart can do with regular playing time at third base, but if things begin to get ugly for Stewart, a platoon may be Dale Sveum's best option.
* With 2010 LD% rates as the baseline.
* I started my analysis with splits from 2008 and on since that was the first year that both players had more than 50 plate appearances against both righties and lefties.
* The 175/450 PA split was determined by the following method. Prince Fielder was the only player who played 162 games last year. He accumulated 193 PA against lefties and 499 PA against righties. The Brewers scored 721 runs, while the Cubs scored 654 runs. Scaling the number of plate appearances down to account for the fewer runs scored, and thus plate appearances, for the Cubs, resulted in the 175/450 PA split. While this method isn't ideal, it's quick and relatively accurate.