Manly five-eighth Kieran Foran admits Glenn Stewart’s injury is a huge blow to the NRL premiers but vowed the Sea Eagles will improve on their patchy form in the back end of the season.

Stewart will miss up to three weeks after partially tearing the medial ligament in his left knee during Manly’s 26-22 defeat by Melbourne on Monday.

Geoff Toovey’s side has been plagued by injuries this season, with Foran enduring a long spell on the sidelines with hamstring and ankle problems, and the Kiwi playmaker says the players must show they can cope without their star back-rower.

“You can’t replace someone like Glenn Stewart, but we’ve all got to step up another level now and cover for him,” Foran said.

“Everyone in the competition knows what a great ball player he is. But he does a lot of tough stuff as well, a lot of cleaning up.

“He’s an 80-minute all-round player and does a lot for us on the right-hand side.”

In addition to Stewart, who also sat out six weeks earlier in the year after dislocating his finger, and Foran, Manly have been without giant back-rower Tony Williams for most of the season due to injury and suspension.

Foran said the constant changes have not allowed the side to be as dominant as they were in 2011, but warned that the best form is still to come.

“It’s halted our momentum this season,” he said.

“We haven’t been able to string together any games with our full side on the paddock either through injuries or suspensions.

“I am quietly confident that if we get our best 17 on the paddock, then we can give this thing a real shake.

“T-Rex has hardly played this year and, when we get him back, he will certainly give us some good go-forward.”