Joe Blanton is slated to miss 3-6 weeks thanks to a strained oblique suffered Wednesday. This late into spring training, these kinds of injuries have an actual impact on your roster, as you can't simply wave away a chunk of the time missed. Blanton isn't an ace by any means, but as an above-average starter (with newfound strikeout rates) on a team most people are pegging to win not just their division, but potentially a third-straight NL crown, you can see why his departure can mess up a fantasy staff's depth. You're missing out on a few potential wins as well as strikeouts, and not in a month you want to see a pitcher missing thanks to colder weather and the like, either.
Given it's just a few weeks (though potentially closer to a month-and-a-half) you won't want to get rid of Blanton if you own him. The more pressing question is how you replace him–there won't be any pitchers of Blanton's quality left on free agency unless you're in a kiddie-pool sized league with shallow free agent waters, but chances are good you were counting on those April innings to come from somewhere. Kyle Kendrick is the Phillies most likely replacement for him–we had him pegged for 48 innings as a spot starter (and 75 innings overall) in the depth charts, innings it looks like he'll pick up right off the bat. The numbers are not anywhere near Blanton's, and likely won't do much to help anyone, even in the deepest of leagues: 4.80 ERA, 1.45 WHIP, 4.1 K/9. He may pick up a couple of wins thanks to the Phils excellent offense, but you may be better off taking another approach to replacing Blanton.
If the starters aren't there on free agency to replace Blanton, see what you can do about creating a two-headed relief pitcher monster in order to snag some of those innings, strikeouts, and maybe even a win or two that you were expecting from Blanton. Many top relievers who have dreams of closing out games are selected at auction, but there are plenty of excellent middle relievers who sit around all year on the waiver wire, never to be selected, because the league you are in doesn't count holds or saves. Someone like Jason Bulger, who is just in the middle of the Angels pen, but is slated to have 67 strikeouts in 60 innings with a solid ERA, is the kind of player you could look for. Daniel Bard isn't expected to take saves from Jonathan Papelbon for a few more seasons, so take his slider and strikeout numbers and forget Blanton is missing. Maybe those two players aren't available to you, but there are more of that pitcher type leftover after drafts than you can count, so start digging to find your temporary replacement(s).