This is an absolute must-win game. Most players and teams in the league don't doubt the Celtics' quality - but you never know what Celtics team will show up (unless it's a big game when they will be up for it). Great teams put away inferior teams, they don't fall behind by 20 due to being lackadaisical and then have to spend the whole game catching up.
If the Celtics want to be in the conversation as a possible Finals team, then these games are must-wins.