Hayward, who did not play in Saturday's preseason finale against the Cleveland Cavaliers, told reporters Wednesday that his back has bothered him in the past but that he would typically recover quickly.
This time, the soreness lingered, Hayward said.
"That's what made me feel like I needed to dial it back a little bit," Hayward said. "In the past, it's bothered me but always calmed down and was fine with treatments and stuff. But this time it was getting worse, especially after playing."
Hayward expects to be on the floor Tuesday against the Philadelphia 76ers, but he understands that it's going to take time for him to be back at full strength after missing all but one game of the 2017-18 season.
"I have found out that is going to be a process because of not playing for so long," Hayward said. "The rest of my body has to readjust to playing."
Hayward broke the tibia in his left leg and dislocated his ankle on Oct. 17, 2017, and missed the rest of the season. An additional surgery in May to remove a plate and screws slowed his offseason preparation.
Last week, he told ESPN's Zach Lowe that the delay affected how he feels ahead of the start of the season.
"Physically, I feel pretty good," Hayward told Lowe. "But I don't feel comfortable on the floor yet. It's one thing to be physically able to do everything. It's another to be a basketball player -- the timing and the rhythms. When you've been playing a long time, you just know. But not after you take a huge break and come back to a new system."
Hayward played in three of the Celtics' four preseason games, averaging 19.7 minutes before sitting out the finale.