Hayward checked in with 5:06 left in the opening quarter.
Boston coach Brad Stevens told reporters earlier Saturday that he anticipated Hayward would play but that Hayward would make the call after going through warm-ups.
"I anticipate he'll be available, but that's his call," Stevens said.
In coming off the bench, Hayward's minutes will be monitored, Stevens said.
Hayward has missed a month with a right ankle sprain but has progressed in recent days to the cusp of a return on Saturday, with the Celtics trailing the Heat 2-0 in the best-of-seven series.
The Celtics upgraded his status to questionable for Game 3 on Friday.
Hayward has had an outstanding season for Boston -- averaging 17.5 points, 6.7 rebounds and 4.1 assists -- and his return would deliver some desperately needed firepower from the Boston bench. Hayward has shot 38% on 3-pointers this season and 50% from the floor.
Hayward, 30, suffered the ankle sprain in Game 1 of Boston's first-round series against Philadelphia on Aug. 17 and left the Disney Wide World of Sports campus to rehab before recently returning.