Independent of this Finn tweet, Bill Simmons said he felt Kyrie quit in the Bucks series in the BS podcast with his dad (posted yesterday). Also, there are those reports Kyrie was doing odd things like requesting to guard Giannis and predictably getting destroyed.
I see more than a few people quick to defend Kyrie in this thread so far, and I think it's because "quit" is such a strong term, especially in sports. It almost seems to suggest intentional tanking of a play, which is not what I think happened, but if you tell me his heart wasn't totally into it (especially when the Celtics fell behind) I would agree with that statement without any reservation.
Even something as simple as taking more pullup 3s because you don't want to spend the extra effort to drive or get hit can be an example of being checked out mentally. Not to mention, not wanting to get injured because you know you're not going to re-sign leads to more cautious play.
I just really wish Stevens had benched him at the end. If you're going to lose the series anyway, play the players who are actually listening to the coach.