Sam ruptured the tricep muscle in his left arm during the 22-12 defeat by the Melbourne Storm at Mt Smart in Auckland on Sunday, an injury which requires surgery and a three-month lay-off.
In his place comes Steve, who is returning from injury himself - an ankle fracture - and he is a player keen to make his mark in his first game for the Warriors this season.
Steve Rapira, a converted second-rower, has added 10kg to reach a playing weight of 103kg and his front-on defence is the best around according to coach Brian McClennan, who was disappointed with that aspect of the game against the Storm.
"When we scored against Melbourne we had to score around their defence,'' said McClennan. "They scored all theirs coming straight through us and that's not good enough.''
Of Rapira, 23, he said: "He's a very aggressive defender, and his work rate, we need that through the middle of the park. I thought Billy Slater was terrorising us there on Sunday. When Steve comes on, he'll help tighten things up in that area.
"He's as hard a front-on defender as I've come across in the game. If anyone wants to take him front on, head on, well good luck to them.''
Rapira, who began training four weeks before his teammates in bid to bulk up, only to fracture an ankle in a pre-season game, said his big brother was a source of inspiration and also education.
"I was never as big as him so I didn't have as much of an impact so I guess I had to rely on a lot more technique, whereas now I've got the size so we'll see if I can do some damage.''
He has played four matches with the Auckland Vulcans in preparation and said he had felt the difference his bigger size made on the field and in recovery.
The game against the Penrith Panthers on Monday night is guaranteed to have extra sting given it is the first time the Warriors have come up against their former coach Ivan Cleary.
Adding to the enormity of the match is the fact the Warriors desperately need the win to stay in the hunt. Their away form is poor - their only win away from Mt Smart came in round two against the Parramatta Eels, a record of one from five.
The Warriors are ninth, two points from the Queensland Cowboys. The Panthers are in 14th place.
McClennan was in no doubt that Cleary would be putting his inside knowledge to good use, but the Warriors would also know what to expect from a side coached by him.
"He would have some advantages with his knowledge for sure. We'll find out who he will pick on.
"Our players all know and respect what Ivan will do and has done in the past. He leaves a great legacy at this club as a player and a coach.''
McClennan welcomes back fullback Kevin Locke from a sternum injury suffered in round four. He is a replacement for Glen Fisiiahi, who has a knee problem, and his return is timely, said McClennan.
"He's a cheeky little bugger is Lockey and he plays like that so he's got a box full of tricks. If he can play the 80 [minutes] it will really help us a lot.
"We've really suffered from not having a consistent No1 and it was just highlighted on Sunday wasn't it. You look at Billy Slater in the No 1 jersey for the Storm and he was so comprehensive and so threatening. it would be nice for us to head that way as well.''
