Reading more about the recovery time between Grade 2 ankle sprain (4-6 weeks) and Grade 3 sprain (several months), the Celtics report that he sustained a Grade 3 sprain and is expected to miss 4 weeks makes no sense. A Grade 3 sprain involves a complete tear of the ligaments, and is often followed up with surgery.
I'm hoping that Hayward only sustained a Grade 2, which matches the projected time table for his return of 4 weeks (2 and a half weeks to go).
If he really did sustain a Grade 3 sprain, there is no way he would be coming back for the playoffs; his season would be finished.
Hope one of the reporters clears this up by asking the team at some point.