Despite evidence to the contrary, the Cubs called up Double-A shortstop Starlin Castro today in order to bump Ryan Theriot to second and get Mike Fontenot out of the lineup. It's a strange move—Castro has 109 at-bats at Double-A, and Fontenot's main issue is a lack of power (something he normally has plenty of for a middle infielder) but in just 77 plate appearances—but it's one that can benefit both the Cubs and fantasy owners.
Castro, who turned 20-years old in March, hit .376/.421/.569 in the Southern League this year and projects to be a star level bat, according to Kevin Goldstein. Chances are good he is available unless you are in a super-deep league because, as stated before, he was not supposed to be called up this time of year, and while he had an impressive line in 2009 for his age at Double-A, he didn't look like someone that was about to be summoned to the bigs, either. Fantasy owners will take any help they can get at the shortstop position though, and as a bonus, Ryan Theriot will now be eligible at second base and shortstop.
If you are in a keeper league, then Castro is definitely someone you should add while he's inexpensive and freely available (assuming you aren't in a league that has ridiculously deep MiLB rosters, anyways). Even if he struggles to find his power at first in the majors, the barrier for entry at shortstop is so low that if he were a league average hitter he would be a decent option in mixed leagues.
Theriot was a three-star shortstop and has started 2010 as he normally does, though with a much higher batting average than we will see by year's end. He fits in at second base in NL-only or very deep leagues thanks to his steals, as he is as much of a source of power as black holes are a source of light or Dane Cook is a source of comedy. If you already have him at shortstop, he's probably best left there though, unless you got stuck with a poor second baseman.
The only people losing out here are Mike Fontenot and the people who drafted him to play second—Fontenot's start to the year isn't great, but he's making contact and isn't striking out. Last year his below-average BABIP killed his line, but his .141 ISO showed some promise. This year his ISO is at .042 (though with just 77 PA, it is nowhere near stabilizing either) and his BABIP is at .323. He would make a fine second baseman if he could get those two numbers to work at the same time, but the Cubs don't want to give him the chance with Castro waiting in the wings. Fontenot should be dropped, as he's not so valuable that he needs to be stashed on the bench in case Castro fails to impress in his first call-up.