Di Resta is not your run of the mill rookie. He's not wet behind the ears perez or Maldonado, he's been around the team and the cars, and he's a DTM champion. He wasn't born yesterday and he knows the ropes.
Secondly sutil still appears to me as the better driver overall if we wipe away all the BBC bias. There were times when diresta seemed lost on track.
Di Resta is good, but i don't think Sutil was showed up in any way. His strong finishes at spa and other races still proves he one of the fastest guys on the grid.
His wet weather driving is still among the best too.
If anything they are about equal in speed and sutil has more experience and a little more composure.
Di Resta also has Hamilton Sr. as mentor, which counts for a lot.
I don't think Ferrari will turn down Adrian Sutil if the opportunity arises. As it is if we look across the driver market, Sutil is the best guy who can replace massa. The others are washed up, or ferrari kicked them out (kimi).
Massa wasn't anything special when he joined ferrari. Replacing him with a half decent and hungry driver would be an improvement.