Why wouldn't he be upset with Danny? He massively outperformed his contract, played at an All-NBA level, and played through physical and emotional pain while aggravating an injury that will likely cost him many millions of dollars. He was the embodiment of a leader and a Celtic, and never complained even though the training staff possibly / probably misdiagnosed his injury. He was the team's head recruiter and a face of the franchise to be proud of.
Despite that, neither he nor his agent were even given a heads up that he was being shopped.
Who wouldn't be salty in that situation? Despite being treated like a damaged "asset", rather than a human being, he still has expressed love for the city, fans and teammates.
1. Players have break out seasons and outplay their contracts all the time. If a better and younger star comes up, vast majority of the time, a team will trade that player. That's the nature of the game, and it's why most players when they get traded will say things like "I wanted to be there, but trades are part of the game and its out of your control".
2. There is no evidence to indicate the doctors misevaluated the injury - there is lots of heresy, and that's about it. If the injury truly is worse then projected, then you can Blame IT as much as anybody - he's the one going around telling the world he's fine moreso then anybody.
3. Who says he was being shopped? I strongly doubt Danny was calling up teams saying "hey bro, what will you give me for IT?". I don't imagine he had any intention of trading IT - but Kyrie demanded a trade out of Cleveland, and Danny Ainge happened to be the GM of a team with lots of nice assets...so when Cleveland came calling offering a younger, better superstar in return for the ageing, injured, expiring Isaiah Thomas, Danny couldn't say no. It's like my current car. I have an Alfa 147 GTA which I absolutely love, and it's not for sale - I have no desire to sell it. But if somebody stops me randomly and says "hey I'm bored of my Audi S5, want to trade?" - I wouldn't be able to say no. As much as I love my car, I can't say no to having a new, better car that I also really love.
I've always appreciated IT as long as he's been here, but I cannot recall the last time I saw a star player traded from a team and react to it in such a vocal, whiney and borderline obsessive way. Even when Pierce and KG got traded - you think they didn't love Boston at least as much as IT did? You they didn't go through as much as IT did playing here - the amount of pain KG had to go through alter in his career just to get on the court and he'd still give it everything he had every night. Guarantee they both wanted to retire Celtics. But when they got traded they didn't speak a word of hate or negativity towards Danny, Doc, or the organisation. All they did is talk about how much they loved Boston and it's fans and the organisation.
Likewise when Rondo got traded - he was the main man on this team for how long? The face of the franchise for how long ? He got traded out for what (at the time) seemed like scraps, he never complained.
Avery Bradley, the longest tenured Celtic, somebody who gave everything he had every night..he got traded by Danny to a team that might not even be a playoff team, and he got moved for what reason? So they could clear a measly $2m in cap space to sign Gordon Hayward. That's like telling AB his loyalty to the team is not worth $2M. But he's not out there whining every day like IT seems to be.
Not saying he doesn't have the right to be upset, just saying that he seems to be one of the only pro players out there who can't seem to remain professional about the whole situation. The last time I saw a player throw hissy fits of this magnitude was Ray Allen, who in hindsight seems to be a bit of a head case.
Seriously, if you look at some of the things Thomas has said via tweeters and the like since the trade...he really does come across as a bit of a head case himself.