I don't think any one single pitcher is going to replace what Clemens gave the team last year, other than maybe Clemens himself. His W-L record aside, he gave them 200+ innings of quality pitching. It's unreasonable to expect a rookie to go out there and come close to matching what Clemens produced on the mound.
Realistically, it's going to come down to what Backe, Rodriguez, and the bullpen can do over the long haul that will influence how this season turns out. Although, 6 games in, the offense isn't too shabby. With any luck, they keep up the pace and offset any pitching struggles that arise. If Buchholtz has a solid rookie year he will have made his contribution.