Valsecchi is supposed to be a decent driver, but jumping into a big team mid season would be tricky. I don't see any of the other teams letting their drivers switch. Di Resta comes to mind, but that wouldn't be until the end of season.
It would have to be someone outside of the current grid of drivers. Is old man Barrichello available? Or maybe Senna?
Petrov.
Edit: To back that up (I'm only half-kidding), they need someone who can bring the car home, ergo someone who has experience with F1 and not another potential accident-prone GP2 driver. Not wins, but to get Constructor's points.
I could see Petrov getting the seat if he brings lots of money with him. I don't however think he's the right driver.
Kovalainen. He has already driven in few practices this year for Caterham, so he also has experience from this year unlike the others. And he has always been a quite stable driver excluding the early years in McLaren. Though at least Finnish media is sure he will replace either Pic or Garde in some point of this season, so I'm not sure if he could just jump to Lotus.
Kovalainen would indeed be a great choice. He is a reliable albeit somewhat slow driver. Also he would be a great team mate for Räikkönen. The only problem is that he isn't willing to bring sponsors with him. Which is probably key for Lotus to sign a driver replacing Grosjean and all that french money.