Kahn said it would have been "inconceivable" in the past for the national team's goalkeeper to be a reserve for his club side.
"It's a problem, Jens knows that as well. It's a dangerous thing, a dangerous path," Kahn said at Bayern Munich's training camp at
Kahn, 38, was Germany's first-choice goalkeeper until losing his place to Lehmann ahead of the 2006 World Cup in Germany.
He said Lehmann was good enough to be be in goal for Germany despite losing his place in the Arsenal team, but the lack of match practice could be a problem against top sides at the June 7-29 Euro tournament in Austria and Switzerland.
Lehmann, 38, has turned down the chance to move to Borussia Dortmund for the rest of the season, citing personal reasons.
"It surprises me that he is not seeking every opportunity to get match practice," Kahn said.
Kahn, capped 86 times by Germany, is retiring at the end of the season and said he would be taking some time away from football for a while and "perhaps for ever."
He is hoping to win his eighth Bundesliga title, a sixth German Cup title and a second UEFA Cup winner's medal before ending his playing career.
"I've had a lot of success, a lot of failures, a lot of emotional experiences. You try once again to give everything in the next few months - and then it will be enough," he said.