Injuries happen every year. Even last year Milwaukee lost its starting SG in the 3rd playoff game and was without Giannis for 2 full games in the ECF and yet it still won the title because it is so well constructed (plus they have Giannis). Middleton being out is a major blow to Milwaukee. If both teams were fully healthy Milwaukee would have won this series already as they are simply a better team with BY FAR the best player on the floor. Boston needs to win this game as that is its best shot a title in over a decade. They have to capitalize on Middleton being out.
Middleton is clearly a bigger factor than Rob, but when Rob is really cooking, the Celtics team defense is historically great. The gap is there, but it isn't as big as you imply. In Middleton's absence, they have gone huge, and it has been a problem. With RWill and Middleton both playing, that advantage disappears.
I'd also point out that Middleton's replacements, have been outstanding. The super huge lineups have been a problem and Connaughton and Matthews have shot unusually well. I'm not so sure Middleton would be so great against Tatum and Brown straight up now that they are entering their prime.
This Milwaukee team is built in the classic fashion to win a title. They have the game's clearly most dominant player, a very good coach and a team built to support him. That this Celtics team looks even stronger is an amazing thing. If we win today, we should all realize just how great a group this is historically. Team's without a top 3 MVP candidate do sometimes win, but almost always because the dominant players at that time haven't separated themselves from the pack.
We are going toe to toe with a historically dominant player in his prime, someone with a title under his belt, he's playing spectacular basketball and we still may walk off as winners. We've got quite a team.