Have a look at Massa's results before and after the accident. Before he was regularly getting podiums and race wins. He's not won a single race since the accident, now - okay - Ferrari haven't been as hot 2010 and 2011, but they've still notched up wins and good points. Massa hasn't.
So, yes, it's speculation but dismissing it as "pure" speculation is ignoring the facts. Massa, whether because of the accident or for another reason, hasn't performed since his return in 2010 to the levels he managed beforehand. The natural conclusion is that something about the accident, whether the faintest hint of extra caution or lasting damage from his injuries, has caused that decline.