Sixers’ Joel Embiid (knee) is doubtful to play in Game 1 against the Celtics

All signs point to 76ers center Joel Embiid being unavailable against the Boston Celtics at the start of the Eastern Conference semifinals on Monday.

“If I was betting man, I would probably say doubtful for at least Game 1, but we’ll see,” Doc Rivers said.

Game 1 of the semifinals is scheduled for 7:30 p.m. Monday at TD Garden.

Embiid has yet to practice after spraining his right knee on April 20 in Game 3 of the his team’s first-round series against the Brooklyn Nets.

“He was wasn’t able to,” Rivers said of Embiid practicing on Saturday. “Obviously, we were hopeful for today.”

» READ MORE: What’s Joel Embiid timeline to return? What steps should he take? A local surgeon weighs in.

Embiid sprained his lateral collateral ligament when he became tangled with Cam Johnson and fell to the court while defending the Brooklyn guard on a third-quarter play. He remained in the game, but an MRI the next day revealed the injury.

Embiid was unavailable to play April 22 in Game 4 when the Sixers closed out the opening-round series in a sweep. The Sixers were off Sunday and Monday. On Tuesday, Embiid watched film with his teammates. Then he did some light shooting on the side during Wednesday’s practice. He was evaluated by the doctor Thursday, and the team had off Friday.

“He has to get better,” Rivers said of what will determine his return to play.

Embiid averaged 28.0 points, 12.0 rebounds, 3.8 assists, and 1.7 blocks in 22 career games against the Celtics. In this season’s four meetings vs. Boston, he averaged 36.8 points, 11.8 rebounds, 4.3 assists, and 1.8 blocks.

With Embiid sidelined with a sprained right knee, Reed started at center in the Sixers’ Game 4 victory over the Nets. Reed ended up with 10 points, a career-high 15 rebounds — including eight offensive boards, tying a career high — along with three assists, one steal, and a block.

He’s prepared to step in if needed for Embiid in Game 1.

“I keep the same mental and physical preparation as I would even if he was playing,” Reed said. “I don’t change up what I’m doing any differently. I keep my same mental focus.”

But his Game 4 performance was good for his confidence. Reed said he knows that he can step in and provide valuable minutes for the Sixers.

“Now, I feel like I can go into the next series and give us another boost if Jo is not playing,” Reed said. “I’m confident that I can come in and give us a lot of energy, and help us win.”

This isn’t the first time Embiid has missed time in the playoffs. Of the Sixers’ six consecutive postseason appearances, Embiid was only healthy in 2020 — and that was in the bubble when teams had a four-month hiatus.

» READ MORE: With Joel Embiid out, Paul Reed stepped up when Sixers needed him most: ‘I know what I can do’

Last season against the Toronto Raptors, Embiid suffered a right orbital fracture and a mild concussion during the Sixers’ series-clinching Game 6 victory in Toronto. The injuries occurred when the Raptors’ Pascal Siakam inadvertently elbowed Embiid in the face while driving to the basket late in the fourth quarter.

Embiid, who was already playing through a torn ligament in his right thumb, missed Games 1 and 2 of the conference semifinals against the Miami Heat.