I think anyone who watches can see what an impact Carrick made, and not just link up. He's positionally very sound, and as the season went on got much better at plugging passing lanes. My only concern of him and Hargreaves together would be that they'd both stay back, but just towards the end of the season he started getting forward. He's not fast, but he's big and strong and if he were to practice shooting this summer could chip in a bit more. I agree a trio of Scholes, Carrick and Hargreaves would work well, particularly with Fletcher and O'Shea as a back up for any/all of them.
If park is out long term (and really how valuable was he this season?) we're stretched a bit on the wings. Ole can do a job, as can Fletcher, but they're not starters at those positions. Richardson will be gone soon I'm sure, which only leaves Giggs, Ronaldo and Lee Martin. Up front I'm not too worried, Smith, Saha (well for about half a season), Rooney, Rossi and Ole should be alright, maybe pick up another larsson esque figure (Gudjohnsen?) as support. Think we'll go for a spread scoring approach again next year, rather than the Ruud model...
Defense and keeper wise we're fine. In good shape to defend the title i'd say!