"There was a bit of a tear in his groin and with a bye next week it was a good game to rest."
While hooker Glen Buttriss said Wighton's running and defensive game would be a huge loss, he had no doubt the 62 NRL game Tupou - who arrived in Canberra from the Warriors last week - would be up to the task.
"(Tupou's) a pretty strong runner of the ball. So I think he'll do a good job," he said.
"It's going to be a good test for us this weekend."
The backline injury list is further compounded by injuries up front, with props Brett White (eye) and Tom Learoyd-Lahrs (foot) ruled out for Sunday's clash, while Dane Tilse (bruised knee) still has to pass a fitness test on Friday.
Meanwhile, Buttriss said he's not overly concerned about rumours the Raiders are in talks with departing Penrith captain Kevin Kingston for the 2014 season.
"I'll just focus on my footy," he said.
"If he's coming here next year I've got to try and perform before the end of the season, hopefully try and hold my spot."
-AAP