The Manly Sea Eagles have revealed back spasms were behind Tom Trbojevic's poor first half performance against the Penrith Panthers.

Trbojevic looked to be functioning at well below 100 per cent during the opening 40 minutes for the Sea Eagles, with the back-to-back defending premiers taking a substantial 32 points to nil lead into the halftime break.

The Sea Eagles, who came into the game off a 32-all draw against the Newcastle Knights in Mudgee last week, were powerless to stop the Panthers as Dylan Edwards notched up a first half hat-trick.

Concerns that Trbojevic could have been dealing with yet another leg injury surfaced, however, when quizzed post-game, head coach Anthony Seibold shut down any idea of that being the case, stating he had back spasms, with an injection at halftime clearing up the issue.

"It was his lower back," Seibold said.

"He had a few spasms so had a needle at halftime, but just talking to him after the game, he was fine during the second half.

"He had some spasms, and I'm not sure how that occurred or why. It certainly wasn't his hammy. He is just a little bit sore, but he's fine.

"I wasn't aware of it until halftime, so he had a spasm during the first half. He was able to play through the game, and he was disappointed with his performance, but he isn't the only one."

Some had suggested Trbojevic should have been protected and taken off the field, particularly given the game was gone for the Sea Eagles during the second half, but Seibold claimed Manly's medical staff made no such call, and there was no risk of damage by Trbojevic playing through the pain.

"The medical staff didn't see any real issue. If he was in danger of hurting himself further, we would have looked after him, but there was no advice to bring him off the field," the coach said.

Captain Daly Cherry-Evans said Trbojevic simply wouldn't come off the field unless he was 'done'.

"Tom's a competitor. He is always going to push through, and even to the question about him being pulled off, he isn't going to come off willingly unless he is done. He might have been a little bit off tonight with what's popped up, but it doesn't sound like it's serious, so he will be right for next week. We have a big game next week," Cherry-Evans said.

Trbojevic, who scored a late consolation try, finished the game with 98 running metres from 11 carries to go with a pair of tackle busts.

The loss leaves Manly in seventh spot on the table with two wins from five games, but likely to fall another spot by the end of the round given games remaining across Sunday and Monday.

Seibold's side, who have let in 119 points in their last four games, will face the Melbourne Storm at home next Friday.