The former AC Milan striker's deal expires in June.
“As far as we were concerned Mario was leaving, so he missed his pre-season," Rivère explained to RMC Sport.
“I think every player who didn't have a good pre-season is struggling in the first part of the season.
“At this point he has regained all his physical qualities, but he's not scoring. That's made him tense, the poor guy.
“Mario hasn't lost his talent, the day he gets back to what he is everyone will line up behind him.
“An extension? No, I think it will be over [when his contract ends in the summer]."