Was the second sacking really unjustified? Can he really feel like he was hard done by?
A bit, I think. Surely he doesn't take that much of the blame; Chelsea fared little better after his departure. It's a club that's so difficult to manage that it's matched in this regard only by Real Madrid. The overbearing owner, the questionable dressing room culture, the difficult league where anything can happen -- I'd say that these things should afford a manager more than half a poor season before getting shunted out, at least when he won the bloody league the previous season.
Looking at the bigger picture: do Chelsea really want to be a club where you can win the league and then get sacked halfway through the next season? How is that sustainable? Are there people at the club who genuinely felt that they needed a better manager than Jose Mourinho at the helm? It's hard to find justification for it.
He had a bad season, but I think anybody would. The job is already harrowingly difficult to begin with because you're working under a notoriously meddlesome owner, he didn't seem to get what he wanted out of the transfer window, and then the players obviously failed him. Who would succeed under those circumstances?
There are basically two scenarios here, two unspoken statements from Chelsea, that are both equally baffling: either it is inexcusable for a manager to have even half a bad season, even right after winning the title; or they believe that Jose Mourinho spontaneously became a shit manager over the course of his summer break. It almost has to be one or the other.
Considering his history of wanton success and his role in Chelsea's rise to the top, I think Mourinho deserved more than to be tossed aside at the first sign of struggle. It certainly doesn't seem remotely believable to me that Chelsea struggled because Mourinho isn't a good enough manager. That would be so baffling a claim that his sacking really must stem from something else, but while the Eva situation wasn't so pretty, that hardly seems enough to do it.
What, then? The only other thing I can think of is that Roman Abramovic is an impulsive malcontent who has a long history of doing exactly this whenever something looked a bit tough. Sacking the manager has always been his first reaction, and while his money has certainly elevated Chelsea to the land of the elite, he also seems to be the factor that prevents it from really stabilizing there and manifesting itself as what Mourinho would call a giant club.