I have my doubts a staff of Gallardo, Parra, Suppan, Bush and Looper will be better than average rotation.
You shouldn't, the only real question is how good Parra will be. People tend to overrate the average rotation.
Lets say every pitcher puts up their career ERA this season, the rotation looks like this.
Suppan - 4.66
Looper - 4.40 (only counted starting ERA)
Parra - 4.57
Gallardo - 3.49
Bush - 4.43
The average SP in the NL last year had an ERA of 4.41. So it is a rotation of league average starters with Gallardo being above average and Suppan likely below average. If Parra breaks out or Bush keeps doing what he has since the all star break of last year when he changed his mechanics some then they are above average most likley.