A lot, obviously, but how much? I'm going to do some back-of-the-envelope calculations. I'm no mathematician, and I'm making a fair number of assumptions here, so what follows shouldn't be taken to be a serious projection of what will happen for the rest of the year. It's just an attempt to put into perspective what Wilson's return actually means.
Wilson should return on Tuesday as the Pirates face the Reds. The Pirates will by then have played 50 games, a hair less than a third of the season. Before the season, PECOTA projected that Wilson would hit .267/.319/.397, well below his 2007 totals but better than his performance in the two years before that.
Let's assume he returns from his injury without ill effects that might prevent him from matching his projection. PECOTA projects a VORP (Value Over Replacement Player, a cumulative stat that compares players to the typical Class AAA replacement or bench player) of 9.9 runs, so we can guess he'll post a 6.6 for the rest of the season. BP has Brian Bixler at -6.5 runs for the season and Luis Rivas at -4.4 runs. Chris Gomez is at +3.2 runs. That's -7.9 runs total, with a number of combined plate appearances before today was exactly the same as Nate McLouth's. (UPDATE: I wrote this before Rivas' two homers today -- obviously, he's just trying to annoy me.)
Of course, Rivas and Gomez will still play some once Wilson comes back, but I think it's safe to assume that Gomez gets the lion's share of those leftover plate appearances, which probably means that the two of them combined will come out at around replacement level for the season. So with two thirds of the season left, let's assume we get +6.6 runs of VORP from the shortstop position. Compared to the -15.8 runs of offense we would've received if Bixler, Rivas and Gomez continued playing shortstop as they have, that's 22.4 runs worth of improvement.
Of course, that doesn't take defense into account. I have no idea how to quantify the defense, but the difference is going to be huge. Of course, you've seen our shortstops pile up errors this year, so you didn't need me to tell you that, but keep in mind that BP's fielding metrics pegged Bixler and Rivas as bad defensive shortstops in the minors last year. I don't know what they're going to say this year, but our shortstops are pretty clearly as responsible as anyone for the Pirates' Defensive Efficiency, which ranks third-to-last in the majors. Guessing that Wilson's glove would be ten runs of improvement over Rivas, Bixler and Gomez so far seems extremely conservative, but let's go with that.
Added to 22.4 runs of improvement on offense, that's 32 runs better over the rest of the year, which is three full wins. That's an enormous difference.