The Swede missed Barca's 0-0 draw at Valencia on Saturday after injuring his knee on international duty and Guardiola said a final decision would be taken on Tuesday.
"I hope he can play but the injury is in a place where you have to proceed with great caution," Guardiola told a news conference at the Nou Camp before the Group F clash against the Russian side.
"He has pain in a lot of places and he can cope with it surprisingly well. But when he says it's enough, it's enough."
Even though the European Champions are top of the four-team group after an away draw with Inter Milan and a home win over Dynamo Kiev, the situation remained "tricky", Guardiola said, and victory over bottom side Rubin was vital.
"It will be an important step toward getting through if we win tomorrow," he said.
"We will be three or four points away from qualification for the knockout round and it's very important to beat Rubin considering the other matches we have coming up."
Guardiola defended his Argentina forward Lionel Messi, top scorer in last season's competition, against critics who have said his performances for his country are not of the same calibre as those for his club.
"To me he is the same as always," he said. "He is the same player here as he is in Argentina and he is just as likely to have an off day here or there.
"As far as I am concerned he can play badly as many times as he likes. He'll still be the best."
Rubin's Spanish central defender Cesar Navas, who played for Malaga and Racing Santander before moving to Russia, said he had not given coach Kurban Berdyev any help in preparing for the match.
"We see a lot of Spanish football on TV in Russia and the coach knows exactly what to expect," Navas said.
"He is always perfectly well prepared for our matches in Europe and in that sense he didn't need any tips from me."
