The Boston Celtics did not need Jaylen Brown to win game one, but you cannot like their chances at winning this series without him. He was a true game time decision in game one, with a hamstring injury that has never looked too serious.
Given how close he was in game one, most expected him to return after the extra day off for game two. Now the he has been downgraded to doubtful for game two, you cannot anticipate he will play (Via NBA.com).
There is still a chance, Marcus Smart was doubtful just one day before he returned to the lineup, but the situations are difficult to compare.