I think some of this is not even fair in hindsight. How much of a fit he was is hard to judge given how many people went down, but he scored very efficiently. If the Celtics had trotted out a healthy Jaylen Brown for the playoffs, we might still be playing the Nets, and having Fournier there to provide some efficient scoring would have allowed us to be competitive offensively. He provided something the team had really needed: a disciplined offensive player who's excellent at the catch-and-shoot. Joe Harris plays a similar and equally important role for the Nets. The difference between the two rosters would have still been substantial, but Tatum, Brown, and Fournier is a pretty good offensive trio. And much as I hate thinking about the cliff Kemba is teetering over, if he'd stay healthy too we might have even blown the Nets out once or twice.
My biggest gripes were than he only took around 10 shots a game -- much less than he should have given how much more efficient he is than Kemba and Marcus -- and couldn't handle the defensive assignments he was being asked to handle. I don't think that was on Fournier, I think that was a mix of COVID and trying to fit into a new offensive ecosystem, something he might have had more time to do if he hadn't gone down. And while bringing him off the bench would have been one thing, once Jaylen went down, asking him and Kemba to defend at the point of attack was unfair, but necessitated by all the injuries. The fact that he only took 63 shots would seem to be one that's as much on the offense as it is on Fournier, but I also saw him struggle with the intensity of the face-guarding that comes with the playoffs, so I can see why people would be sour on him.
It was not the best move Danny could have made, but it was one that seemed to strengthen the Celtics at a place where they really needed help: shooting and playmaking to take the burden off Tatum, Kemba, Brown, and especially Smart. The other places where they needed help -- rim protection, for example -- those possibilities were foreclosed when Williams went down again.
In the end, I think there are a lot of players as good as Fournier out there. I think he was a decent fit for a run at the playoffs, but whether or not the Celtics decide to bring him back will depend a lot on what kind of role Fournier wants and how much other teams are willing to pay for a good 3rd option on offense. If the Celtics can bring him back for a similar contract, I think they'll do it, because he's exactly the kind of complementary scorer they need, especially if poor Kemba heads down the drain.