Who will be bumped from the Cardinals rotation when Jake Westbrook returns?

On Tuesday, Jake Westbrook starting his rehab assignment with AA Springfield, throwing 3.2 innings.  He is scheduled to make another rehab start on Sunday (with low-A Peoria) and, assuming he has no setbacks, he could be activated some time next week.

The question on everyone's mind then is: who will Westbrook replace in the rotation?

Assuming there are no more injuries during that time (which, as of late, is not a safe assumption), the choice will likely come down to either Tyler Lyons or Michael Wacha.  While Wacha, who has quickly become a fan favorite, is the sentimental choice to stay in the rotation, the more logical choice would be to keep Lyons.

Here's why.

First off, Lyons has not done anything to deserve a demotion.  He has started 3 games, giving up just one run in each of the first two and only 2 runs before leaving the third game, after which the bullpen allowed two more runs, which were charged to him.  Wacha, while brilliant in his first start, got roughed up in his second start, which is probably a good indication that he still has some developing to do.

Second, the Cardinals have already said that they are going to be cautious with Wacha and limit him to 150 innings this year.  With the Cards in the middle of a pennant race, this could turn into another Stephen Strasburg-type situation, with the team forced to shut Wacha down in September.  If that were to happen, the Cards would need to call up someone to fill in for Wacha down the stretch, such as Lyons, putting a lot of undue pressure on that guy.  It would be better to have that guy in the rotation the whole time.

Third, the Cardinals can still avoid Super Two status for Wacha by sending him down now for another month or two and then calling him up for the stretch run.  He can then be used out of the pen, similar to how Rosenthal was used last year, to help keep his innings down.

Finally, Chris Carpenter is ramping up his bullpen sessions and could start a rehab assignment soon.  Assuming no setbacks, he could return around the end of June or early July.  If that happens (and I admit, that's a big IF), the Cardinals would only need about 5-6 more starts out of either Lyons or Wacha.  Does it make sense to use Wacha for those 6 starts, when it appears that Lyons can be just as effective and we can avoid adding to Wacha's service time?

Both Lyons and Wacha will have one more start to prove whether they are worthy of retaining their spot in the rotation and while I may be pulling for Wacha, in my mind I think Lyons should be the one to stay.