I can't see a reason why they couldn't give it to Senna. if Kubica fell out for a short time, then it should be the reserve driver's role, and if he is long term out, then I think it would be reasonable only if they found one who is better than him BY FAR, if they find someone who's better only by an inch (maybe even has a place as a test driver somewhere) then there wouldbe no point to abandon Senna.
let's see all candidates!
Heidfeld: he's a good driver, but nothing extra, I don't see him turning the world upside-down as Kubi would do. plus, Robert would get a heart attack when they tell him, that his (so far top3 looking) car is given to his
favourite teammate
Hulkenberg: he's not much more experienced than Senna, plus he's got an established place at force india (rumoured that his contract contains a 2012 race seat) imagine if Kubi comes back mid-season, and Hulkenberg's left with no team, no seat for next year. is it worth him?
de la Rosa: seriously. his story is over.
Grosjean: behind Senna on the list, smaller and not as fresh F1 experience, etc
Raikkönen: although him replacing Kubica would make me (and a lots of Kimi-Kubi fans) the most happy from this list(but still sad about Robert, before you come at me!), I just can't see it happening:
1, around september, when renault used Kimi for some press presence, he really got pissed with them
2, I think he invested a lot in Ice1 Racing, I don't think he'd leave that in vain.
3, he now hates f1, and would only think about coming back if they'd offer him Webber's place?
Fisichella: it is true that he drove for renault in 05-07, but I think his f1 career is also done, I can't see renault picking him over Senna.
so no, either of those, I voted for Senna.