With Stephen Drew slow to return from a concussion and without a clear time table, as if often the case with concussions, just ask Justin Morneau, the Red Sox are considering Jose Iglesias at short to begin the season. It all depends on his contribution with the other aspects of his game, so let's see if we can get some rough estimates there. A quick reminder - I'll be discussing value in terms of runs here, but at the end we'll discuss it in terms of wins. The quick and dirty conversion from runs to wins, whether they're runs saved defensively or runs scored offensively, is one win equals 10 runs.
Everyone considers Iglesias the best defender in the minors, and the advanced metrics for his career put him at saving an astounding 41 runs over the course of a full season. Granted, these metrics take much longer than one month to stabilize and this would have put Iglesias as twice as good a defender as anyone else in baseball, but the advanced metrics support the assertion that Iglesias is an elite defender. Last year the best defenders saved just over 20 runs, and since 1950 40 different shortstops have had seasons where they saved 20 or more runs, led by Mark Belanger's 1975 when he saved 33 runs. Given Iglesias' defensive pedigree and universal defensive acclaim, I think 20 runs over the course of a full season is reasonable, and this could be a fairly conservative estimate.
Iglesias is fairly fast, but does not have blazing speed. He seems to have good base running instincts, but does not run very often (12 SB, 3 CS in 400 PA in AAA last year, 75% success rate for his career in the minors). As far as comparables, Rafael Furcal (the current version, not the 46 steal version from 2005) who stole 12 bases and was caught 4 times seems like a good fit. In 2012, Furcal was worth 2.5 runs on the bases. This value includes other aspects of base running, such as advancing on hits, but for simplicity's sake, we'll pick our comparable using stolen base numbers.
Finally, we get to the last numbers on the positive side of the ledger. First of all, Fangraphs defines "replacement level" as -20 runs per 600 plate appearances, so we get to add 20 runs to Iglesias' total. Also, shortstop is an extremely thin position. It has been said over and over again, but the golden age of shortstops is long past us. The average shortstop hit .256/.310/.375 in 2012, so the bar is rather low for Iglesias. That level of league wide production puts the positional adjustment at 7 runs. This means that just by playing shortstop, Iglesias get a bonus of 7 runs over a position like third base, where hitters were about league average, or even more for a position like first base or a corner outfield spot where hitters are penalized for their position, given the expectation that they hit above league average just to be average for their position. So, just for playing shortstop in the majors, Iglesias gets +27 runs to his overall numbers.
So up to this point, we have Jose Iglesias being a 50 run, or five win player, in 2012 if he can just manage to have no negative value as a hitter. For example, Alcides Escobar's 2012 batting line of .293/.331/.390 was worth exactly zero point zero runs. Unfortunately, Iglesias is extremely unlikely to match that kind of production, having hit only .266/.318/.306 in AAA last year. But, in order to have positive value for the Red Sox, he'd only need to be better than -50 runs. According to Fangraph's calculations, Drew Stubbs (-20) was the worst hitter in the league last year. Negative thirty runs is awfully hard to come by, but a nice comparable might be Cezar Izturis' 2010*. Given 500 plate appearances, Izturis hit an incredible .230/.277/.268. Those numbers actually match Igelsias' AAA numbers nicely, adjusting them to the better competition in the majors (.229/.272/.263). Iglesias does have youth on his side, though. He is only 23 and has plenty of room to grow, and this spring training he has six extra base hits in 46 at bats, compared to 11 in 353 at bats in AAA last year.
* The worst batting line of the last 20 years comes courtesy of the Royals, who rolled out Neifi Perez's .236/.260/.303 line for almost 600 plate appearances, good for a mind boggling -42 runs. Sadly, his fielding wasn't doing him any favors either, at -13 runs. This all led to Perez putting up the worst season in the last 20 years at 2.9 wins BELOW replacement and negative eight million dollars worth of performance , and after he was the centerpiece the Royals received after trading away Jermaine Dye to get him. Somehow the Royals managed to foist him and his $4 million salary off on the Giants, where he rebounded to a 1.1 win season, thanks in large part to a return to form defensively.
Overall, this analysis backs up the general assertions that if the Red Sox put Iglesias in the lineup right now, he'd be a two win player. Whether or not that will stunt his development as a hitter or if his failings on offense will affect his focus on defense is an entirely different set of questions. Concussions are tricky injuries and Drew's may linger over the course of the season; if that does happen, the Red Sox may be in the tricky position of balancing using the best shortstop they have on the roster at the expense allowing that player to continue to develop in the minor leagues.