I think every player on the roster had one or more injuries this season. The biggest injury impacts in my mind are:
Jayson Tatum: Just not the same after the shoulder stinger in the Miami series
Jaylen Brown: I'm certain his wrist was never right during the playoffs but I can't prove it and he wouldn't admit during the series.
Robert Williams III: If not for the knee, I think the C's would have #18 right now.
Marcus Smart: Missed time during the season and probably played injured in the Finals
Now, I read a lot of posters saying "but no excuses" which I suppose is true from the coaching perspective, but the injuries do explain what happened, IMO. To make things worse, when players went down, there was not much reliable bench depth to tide the team over.