Anthony Seibold has urged the NRL to reconsider its scheduling after Manly's season was turned upside down by playing three games in 11 days.

Seibold's comments came after a costly night for the Sea Eagles, with Nathan Brown (shoulder), Jake Trbojevic (concussion) and Tommy Talau (leg) failing to finish Thursday's 20-18 loss to St George Illawarra.

The trio are likely to miss next week's clash with Penrith after the Sea Eagles slumped to a third-straight win without Tom Trbojevic.

The prospect of Trbojevic returning next week came as Seibold revealed hooker Lachlan Croker was set to miss the next month of action after having knee surgery.

And while he said his side's play was not up to scratch in the loss to the Dragons, Seibold was keen to underline that the burden of travel was becoming too much.

"This is something I was going to say regardless of the result, but that's five games in 25 days," Seibold said.

"The NRL's never been more demanding and we talk about player welfare and wellbeing and so on.

"I just don't understand how not only is it five games in 25 days, if you narrow it down a bit more, it's three games in 11 days."

Across those 11 days, Manly played Melbourne at home on Sunday April 6, travelled to play Cronulla in Perth on Saturday April 12 and then hosted the Dragons on Thursday April 17.

Seibold also noted issues with draws for NRL rivals such as Canberra, who have been to Darwin, Las Vegas, and Townsville in quick succession.

"We lose two days in the air to Perth, it's a five-hour trip each way, so there's another two days down," Seibold said.

"I don't understand it. The competition is demanding and it's never been more demanding. The GPS data is through the roof this year.

"So I've got to say I just don't understand that. I think it's something that needs to be looked at.

"We had 20 minutes on the field this entire week, so it's difficult. I've seen other teams with it as well, so it's not just us, it's across the competition and needs to be looked at.

"In the AFL they don't have less than six-day turnarounds."

The latest setback for Croker is a mighty blow for Manly, who have lacked the hooker's guile and control out of dummy half.

"Crokes had a clean out on his knee during the week, so he'll be about a month," Seibold said.

"It's unfortunate because he's a really valuable member of the squad and we've missed him."