"He is not on top physically," Blanc told reporters on Wednesday, one day after Benzema netted the opener in France's 2-0 defeat of Luxembourg in a Euro 2012 qualifier.
"It is a problem. He needs to play more matches (with Real)".
Benzema, who was overlooked for the World Cup and then placed under judicial investigation in France on suspicion of soliciting sex with an under-age prostitute, has been struggling to find his stride in La Liga.
Blanc explained he was not sure he would be able to field Benzema against Luxembourg, three days after the 22-year-old finished a match against Romania completely exhausted.
"I was considering whether he should play against Luxembourg, he was so shattered after the Romania game," Blanc said.
Benzema, who scored in a 2-0 win in Bosnia last month and produced encouraging performances against Romania and Luxembourg, has yet to make his mark at Real, which could harm his future with France.
He has played only five La Liga games this season, being used as a substitute four times.
"He should play three, four matches in a row but he is in a big club and we know how it works in a big club - if you play two games and you are not good, you end up on the bench," said Blanc.
"We are going to try and save him."
Real Madrid coach Jose Mourinho said last weekend Benzema had a lot of talent but needed to find the 'killer instinct' that could make him a great player.
Blanc himself criticised Benzema earlier this year, notably saying the forward had to shed some weight to be in top form.
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