Key midfielders Clement Grenier (groin) and Yoann Gourcuff (ankle) are still working their way back to full fitness, while defenders Henri Bedimo (hamstring), Milan Bisevac (back) and Mouhamadou Dabo (suspension) will also be unavailable.
Bedimo and another defender Lindsay Rose (hamstring) both suffered injuries in Lyon's 2-1 win over Mlada Boleslav of Czech Republic in UEFA Europa League qualifying on Thursday but while the former is out for three weeks, Fournier claimed the latter could still play against Rennes.
"Henri Bedimo is out for a minimum three weeks," Lyon's coach said at a media conference on Friday.
"For Lindsay Rose, we will give him until Saturday to see if he can play. Milan Bisevac is out; he will be available for the Toulouse game.
"[Captain] Maxime Gonalons can step into central defence; we spoke about it on Thursday, but I prefer he played in midfield.
"The injuries? There's no miracle solution to always avoid injury.
"It's a difficult moment. I don't know when Clement Grenier will be back, he's improving, he's on the right track … I can't give you a date. He is not looking at surgery.
"Dabo will be back next week. He is out for this game though. Gourcuff is continuing his pre-season."
Lyon learnt on Friday that they will face Romania's Astra in the final play-off round of the Europa League.
Fournier admitted he does not know much about the club from Ploiesti but argued the first leg at Lyon's Stade de Gerland will be critical.
"We never know if it's a good draw before we've played," the 46-year-old said.
"I would have preferred the return leg at the Stade de Gerland.
"We can imagine that they are another step up from Mlada. They [Astra] eliminated [another Czech club] Slovan Liberec. We all know how good Romanian football is.
"We will have to do the job in the first leg. That will help the return. The match at the Stade de Gerland will go a long way to settling the tie."
