All three star players are coming off season-ending injuries. And if any one of them sits out games this season because of injuries, the Rams’ already sketchy prospects for success are all but doomed.

So there was muted but major concern Tuesday when Kupp left the field with a member of the team’s medical staff about an hour into practice at UC Irvine.

The receiver, who missed the final eight games last season because of an ankle injury, left after participating in a red-zone drill. He was not limping and did not appear to be in physical pain.

Coach Sean McVay said he did not know specifics about Kupp’s situation.

“You’ve got to be able to move on,” McVay said. “I mean, I hope he’s OK. But, I mean, we got to be able to go practice. Guys have to be able to step up.

“And obviously, we’re much better when he’s here and you hope he’s going to be OK.”

Kupp, Stafford and Donald are the Rams’ highest paid-players. A few months ago, Kupp was on track to carry a team-high $27.8-million salary-cap number. The team restructured the deal to give him a $17.8-million cap number.

Donald’s cap number is $26 million, Stafford $20 million. No other player has a cap number over $9.7 million, according to overthecap.com.

So the Rams must protect the investment in their three stars.

Rams quarterback Matthew Stafford participates in drills during the team’s training camp on Tuesday in Irvine.

(Ryan Sun / Associated Press)

Stafford, 35, sat out one game last season because of a concussion and the final seven games because of a spinal injury.

The Rams are proceeding with caution as Stafford prepares for his 15th season.

After a team day off on Sunday, Stafford did not practice Monday because of what McVay described as a scheduled day off.

Stafford returned Tuesday.

“It was kind of planned a while out,” Stafford said. “I was wanting to be out there real bad [Monday] but I know it was the right thing to do.

“As of right now, I don’t think I have many more of those planned.”

Asked his reaction to seeing Kupp leave the field, Stafford sounded unfazed.

“I just want to know what’s going on — that’s the biggest thing, and I’ll figure that out at some point,” he said. “But I trust our guys, and whatever’s going on I know they’re doing everything they can to work through whatever’s going on, and he’ll be back out here whenever that’s the right time.”

Donald appears recovered from season-ending ankle surgery that sidelined him for the final six games last season.

Donald did some individual drills but spent most of the workout in street clothes as part of a rest day, according to McVay.