While Hayward averaged 11.5 points a game this season, it wasn't until the past few games when Hayward appeared to have shaken off the impact of the 2017 injuries.
This calendar year, by way of seven game titles, he'd looked magnificent and the Celtics had been firing on all cylinders.
While there's been no indication of a timetable for Hayward's injury, Jeff Stotts of In Street Clothes recently took a deep dive into hand fractures in the wake of the injury suffered by Stephen Curry.
Kemba Walker add 26 points as Boston (7-1) won its seventh straight game and set season highs for total points, first quarter scoring (39 points) and first half scoring (72 points).
The Celtics say Hayward was sent for an X-ray that confirmed the diagnosis.
Stevens said the team officials and Hayward will consult with doctors Sunday to determine whether Hayward needs surgery.
The Spurs, who fell to 5-4, were led by 22 points from DeMar DeRozan and 20 by Australian reserve guard Patty Mills. Celtics coach Brad Stevens said Kanter felt good after participating in pregame workouts but is still not ready to play.
Spurs: San Antonio's last home loss to Boston was March 31, 2011.