Manly co-captain Myles suffered the injury early on in the Sea Eagles' NRL loss to Penrith on Sunday.
Myles completed the match, but Manly coach Trent Barrett said the injury could spell trouble and the veteran forward has been sent for scans.
"He needs a scan on his knee, I don't know how serious it is," Barrett said.
Myles suffered the injury in the opening minutes of Manly's dramatic 31-24 loss to the Panthers at Brookvale Oval.
He spent much of the first half in the attention of Manly's medical staff, who eventually declared he was fit to play on.