In 2011, Pirate starters pitched 923 1/2 innings. Those innings broke down like this:
Provided by Baseball-Reference.com: View Original Table
This isn't great: it's just about 5 2/3 innings per start on the nose. Given that no one on the Pirates' staff showed much durability before 2011, that's not terrible, but as the rotation started to fall apart in the second half the team did the same.
Let's look at this from the perspective of 2012. The Pirates should be able to pencil Morton and McDonald back in for somewhere between 340 and 400 innings. Both guys saw increased workloads at the big league level in 2011, but neither pitched that many more innings compared to their 2010 combined MLB/MiLB totals. Injuries always happen to pitchers, but at this point these guys are relatively safe bets, as youngish pitchers go.
Beyond that, though, what do the Pirates really have? I don't know if I'd count on another 150 innings from Jeff Karstens; I still don't buy his low ERA as progress and I still have questions about his durability (though this is for another post in the future). Giving Kevin Correia another 150 innings is dangerous. Both his strikeout rate and his homer rate are skewing the wrong way and I'd bet on him getting worse before he gets better in 2012. Ross Ohlendorf is probably a non-tender candidate and even if he comes back, he's better suited to the bullpen at this point in his career. Brad Lincoln started off nicely, but I'm not sure how worthy he is of anything more than a fifth starter spot. No one in the minors (not Jeff Locke, nor Rudy Owens, nor Kyle McPherson) will be ready for a big league spot when the season begins. Brian Burres and Aaron Thompson are filler.
Now consider how relatively healthy the Pirates were this year. How many teams get 150+ innings out of five starters? The Phillies didn't even have that this year. The closest the Pirates have come recently is 2004, when they had five guys with 130+ innings.
So here's my question: how do you possibly get back to 900 innings without Paul Maholm? I'm not sure they can get there with him, but this rotation could be a disaster zone without him. As far as free agent starters go, I'm sure the Pirates will make a big offer to Edwin Jackson and I'm sure that he'll spurn them for less money to stay with Dave Duncan in St. Louis (or something similar to this). If the Pirates sign a free agent, it'll probably be some kind of reclamation project like Dontrelle Willis or Scott Kazmir. Not guys I'd count on big innings from. They could swing a trade, but I'm not sure they're in a place yet to be dealing for someone like, say, James Shields, even if they thought they could extend him.
Maholm's not a great starter and I'm not sure he'll be worth $9.75 million, but he is pretty reliable and I just can't see a way the Pirates can build a rotation without the innings he'll give the club.