Signing an ace like Greinke works fine if you have a constant stream of good pitching prospects that you can ride until they hit arbitration and then replace them with new prospects. Greinke is going to get a $125M/5yr kind of deal. That's $25M per year!!! The whole Twins starting payroll this year was supposed to be ~$30M and that is the highest it has ever been (I think ). That only leaves $5-10M to sign your other 4 starters. So short term Greinke works if you go with Diamond, Gibson, Hendriks, Wimmers, etc... because they are cheap. That philosophy stops working though once you get to arbitration and your starters start earning in the millions.
Even if you are willing to pay the $25M/yr to Greinke and just live off pre-arb years there is still a problem. The Twins don't have the young pitchers to even make it worthwhile. You want those first years of a Greinke contract to be years you think you can go all the way. Here is what a Twins starting staff would look like though:
Greinke
Diamond
???
???
???
Options:
Hendriks
Walters
Gibson (will he be ready to go?)
Cheap Veteran (hello 37yo Pavano!! or hello Marquis part 2!!)
Deduno
De Vries
Who thinks we could go all the way on that? BTW, things aren't looking any better for the 2014 rotation. Or even possibly the 2015. As you can see if you sign Greinke you don't have the money to improve the rest of the rotation enough to make it worthwhile. Here is what I think is a better option: Sign a $70M/5year deal (I like Shaun Marcum personally), a $33M/3yr deal (somebody between Edwin Jackson and Anibal Sanchez), and a cheap veteran (Pavano?) or Blackburn if he turns things around. This would be your rotation:
Player (Age,ERA, WHIP, K/9)
Marcum (30, 3.39, 1.166, 8.4)
Jackson (28, 3.73, 1.135, 6.8)
Diamond
Pavano/Blackburn
Hendriks/Gibson
Then in 2014 you can replace Pavano with 2 out of Hendriks, Gibson, Wimmers. This method keeps your payroll ~$30M/yr, runs out a decent rotation and allows you to have flexibility in the mid-term with only ~15M/yr locked up long term.