Will be interesting to see where Lotus go with a replacement, ideally get next years driver in there for a couple races... But that would mean having to sort out next years driver first
For Lotus, it's simple; they need to decide who's going to get the most points for them in the final two races, with the possibility (however unlikely) of them getting 2nd in the constructors and the money which comes with it, and they've realistically got 3 choices: Valsecchi, Kovalainen and Hulkenberg (with him being out of contract for most of this season.
Hulkenberg is the obvious choice
if they think they can afford him next year, but Hulkenberg has been burned plenty of times now and needs to be sure that he will be there in 2014, otherwise he'll tarnish himself slightly (and potentially be without a seat if Lotus can't afford him and the other seats are filled while he waits).
Kovalainen has F1 experience this year, has experience towards the front and is a safe pair of hands, though will presumably be race rusty and potentially not fit enough (especially with these both being anti-clockwise circuits and him only doing the odd practice session in F1 this season).
It'll be a huge kick in the teeth to Valsecchi if he's overlooked, but the point of the 3rd driver is really to substitute for a driver if he falls ill before final practice, not necessarily to replace a driver with this much notice.
I'm going for Heikki. While Hulkenberg is effectively out of contract, there will probably not be enough time to sort out any contractual issues, then get a seat fitting, and there'll certainly not be time to do a straight-line test (if Lotus could afford it anyway) or get into the simulator to learn the Lotus systems... and I don't think Nico would be up for it anyway - he'll show better in these final two races in the Sauber than he would the Lotus.