The obvious thing that needs to happen for the Twins to contend next year is for Mauer, Morneau and Span to get and stay healthy. But that is beyond anyone's control. While their staying relatively healthy is necessary for the Twins to contend, it is not sufficient.
There has been a lot of discussion about the Twins need for another options at shortstop and better offense from their backup catcher. But the real issue for the Twins, as always, is their pitching staff. Regardless of what else happens next year, without better performances from their pitching staff the Twins are not going to be competitive.
One of the difficulties with evaluating a team's pitching is that it is more than the sum of its parts. The outs a team will need over the course of a season is relatively fixed around 1450 innings pitched (more if the team win a lot on the road, fewer if they lose a lot). That means the fewer outs a team's starters get, the more outs will have to come from the bullpen.
While there are exceptions, in general the more a pitcher pitches, the worse their performance will be. This is true for individual games and, for relievers, over periods of games. Whether it is true over the course of a season is a little more questionable, but it appears to have at least some effect on the bullpen. What that means is when starters pitch only a few innings in a game, the bullpen performance will start to suffer. There is a flip side to that effect that I think we saw happen with the Twins this year. When the bullpen is unreliable, the manager tends to leave starters in longer.
Overall, compared to 2010, in 2011 the Twins pitchers faced more batters and used more pitches for each batter they faced. The result was that they threw more pitches over the course of the season. The Twins starters threw 100+ pitches 68 times in 2011 compared to 61 times in 2010. In short, by almost every measure the work load on the Twins pitching staff was higher last year than the year before. That extra work was likely done by tired pitchers on the downside of their performance curve.
The point of all that is that, like a lot of things in baseball, the relationship between pitching ability and results is non-linear. Bad pitching tends to snowball into worse pitching. So what needs to happen to avoid the same problems next season? Where do the Twins get those 1450 innings pitched without relying on worn out pitchers?
The Twins have two starters set. Carl Pavano and Scott Baker both performed about as expected this year. Pavano has pitched over 200 innings the last two years and can probably be counted on to do the same again next season. Baker pitched well last season until he got hurt. Baker has pitched 200 innings only once in his career. He may do that next year, but I wouldn't count on it. Lets say 180 IP is a reasonably optimistic performance. It appears the Twins will go into spring training expecting Francisco Liriano to be in the rotation, although he could pitch his way out of that role. He can probably be counted on for 180 IP if he stays healthy and is reasonably effective. The fourth starter is probably a competition between Blackburn and Slowey (if the Twins decide to offer Slowey arbitration). Either one could pitch 160 innings even if they struggle, 180 if they are effective.
It sounds like the Twins plan to add another top starter - a guy you would project to give you 200+ innings. With that move, their starters would be projected to give them 940 innings. Diamond, Hendricks, Swarzak and Duensing are also possibilities. But it appears Duensing is going to end up in the bullpen and I don't the others are likely to win a starting spot unless someone else falls on their face. We can expect some combination of those guys to get a few spot starts for maybe 30 IP in 6 starts, many more if any of the starters are injured.
Those 970 innings from starters (that's about 6 IP per start) leave the bullpen to get somewhere around 480 innings pitched. To call the Twins bullpen "thin" right now is an understatement. Glen Perkins is the only reliable reliever returning. Mijares (who, like Slowey, is rumored a possible non-tender) and Alex Burnett are the only other two relievers who were significant parts of the 2011 bullpen. There is a possibility that Nathan will be back or, perhaps even Capps if Nathan goes elsewhere. Duensing, Swarzak, Diamond and Blackburn are all starters who may fall into the bullpen. They also have a handful of minor league arms that could fill in. But after Perkins, there is no one you would have any real confidence in.
In summary, the Twins need an established starter, a closer and at least one setup guy. Those are all much higher priorities than another shortstop, catcher or right fielder.