The Celtics are not playing Sunday but the game between the Milwaukee Bucks and Philadelphia 76ers has important implications.
Currently the Bucks are in first, 1.5 games ahead of the Celtics. The 76ers are in third, four games behind the Bucks and 2.5 games behind the Celtics. After Sunday's game, all three of these teams will have four games left to play in the season.
A Bucks' loss to the 76ers would tighten the race considerably. If the Bucks lose once more and the Celtics win all four remaining games, then the Celtics will be the number one seed. A 76ers' win would also put them just two games behind the Celtics for the second seed and home court advantage should they meet in the second round. That would make Tuesday's game between the Celtics and 76ers critical.
On the other hand, a Bucks' win would essentially clinch the top seed for them. Making up a two-game deficit with four to play would be highly unlikely. But a 76ers' loss would also pretty much clinch second place for the Celtics as they would have a three game lead over Philly.
If the Bucks lose, then I would expect the Celtics to go all in for a chance to get the number one seed. If the Bucks win, I expect the Celtics to use the last four games to ease up a bit and get rested and healthy. That's why Sunday's Bucks-76ers game is so important.