LAKE BUENA VISTA, Fla. -- Boston Celtics forward Gordon Hayward said he knew something was wrong the moment he suffered a Grade 3 right ankle sprain in the fourth quarter of Boston's victory over the Philadelphia 76ers in Game 1 of their first-round series.

"I knew right away," Hayward said Thursday in his first interview since the injury. "I heard it, I felt it, and I knew it wasn't just your casual rolled ankle.

"It was swollen by the time I was leaving the court, so I knew it was definitely worse than normal."

The injury came at a bad time for the Celtics, who entered the playoffs with high expectations for a deep playoff run thanks to a roster that was fully healthy.

That is no longer the case now that Hayward, who averaged 17.5 points, 6.7 rebounds and 4.1 assists during the regular season while shooting 50% from the field and 38.3% from 3-point range, is out for at least four weeks. That will keep him on the sideline through at least a possible second-round showdown with the defending champion Toronto Raptors -- and with no guarantee he'll be ready to go later in the playoffs, if Boston is able to make it to the Eastern Conference finals and beyond without him.

"As far as timeline, it's kind of up in the air," Hayward said. "It's kind of just how my body responds to the rehab and all that stuff. So it's hard to say. No one knows, really."

What Hayward and the Celtics do know is that he won't be back any time soon. Because of that, Hayward said he is looking at the possibility of scheduling the impending birth of his fourth child during the time he is out.

The Celtics had been prepared for Hayward to leave the team for a few days at some point in September to be with his wife, Robyn, for the birth.

"I think it's two separate things," Hayward said, referring to his injury and the impending family addition, "but it's something I'm taking a look at. It's just very unfortunate of the timing of the whole deal, so I think I'm just trying to take it day by day, and we're looking at scheduling for the baby and all that stuff, so it's taking it day by day."

Hayward's time with the Celtics has been defined by injuries. After signing a four-year max contract in 2017, he suffered a gruesome left leg and ankle injury six minutes into his first game with the team that October. He missed the rest of the 2017-18 season and was less than his former self last season as he began to knock off the rust that came with a year of rehabbing and not playing.

This season, Hayward has been terrific for the Celtics. But he got his left hand caught in LaMarcus Aldridge's jersey and fractured a bone, causing him to miss a month. He also missed time in December with nerve pain in his left foot -- the same one he injured two years earlier.

While Hayward, 30, did manage to joke and say he could use some "Disney magic" to heal faster when asked if there were benefits to rehabbing inside the bubble at the Walt Disney World Resort, he admitted the mental strain of rehab is a bigger challenge than the physical act of trying to get back on the court.

"It's hard," he said when asked about the string of injuries. "It's hard not to get into that. I think teammates, family, mentors, having good people around you, I think helps with all of that, certainly. Like I said, I just try to take it day by day, honestly, and just trying to attack rehab. I've definitely been here before, so I know how to do that.

"The mental side of rehab is by far more difficult than the physical side of rehab. There's a lot of time when you are alone and a lot of time when you are contemplating, a lot of time to think. The mental side is the hardest part. I think having good people around you is very key, it's very important, and I definitely have that."

Although it was frustrating to have to watch his teammates play in Game 2 without him, Hayward said the way Boston's bench players stepped up and helped the Celtics blow out the Sixers and take a 2-0 series lead did take some of the sting out of it.

"I mean, happy that we played well," Hayward said. "The nice thing about our team is we have a lot of talent. A lot of guys that can pick up the slack, so that's definitely a plus. I'm happy to see us playing well. I think we were playing our best basketball heading into this, so hopefully that we can continue that."