I kind of agree with Giblet, Massa's best days involved TC and driver aids (safe for 2008 and part of 2009). For me his best days were late 2006 to Hungary 2009. He was never the most skilled driver but he was feisty, always challenging, pushing as much as he can (his fight with kubica in the rain at japan in 2007 was epic). He also went down to the wire with Hamilton in 2008 and with Raikonnen as his team mate he never looked bad (or terrible).
But we have to be honest, his achievements came in car that was supposed to win, the Ferrari's of 2006-2008 were mostly carry-over cars from ferrari's championship winning years with Schumacher (Ferrari won all but 3 championships between 2000-2008). But F1 is a different environment today, it isn't just Mclaren and Ferrari as it was then, now there's 1 second separating the top 7 or 8 cars, the field is tightly packed and Ferrari don't dominate as they did before. Alonso has absolutely juiced every inch out of that car for the past 2 seasons (I don't believe the F10 was anywhere near the RB6 and we all know the struggles of the f150 and now possibly the F12) massa just can't keep up, why? Who knows, its just speculation.
Possible replacements,
-Perez (not likely for this season, under contract w/ Sauber)
-Alguesuari (possibility)
-Sutil (last year he expressed interest in Ferrari and said he'd gladly take a #2 role)
-Bianchi (under contract with FI)
-Kubica (Health??)