Bundesliga - Lehmann suing over 'Muppet' jibe
The 41-year-old, who recently rejoined the Gunners to help ease an injury crisis, had criticised Werder Bremen keeper Tim Wiese while commentating on the Bundesliga side's Champions League clash against Tottenham last September.
Wiese took the criticism badly, hitting back in German newspaper Bild the day after.
"He should go on the Muppet Show," Wiese said. "That man should be on a couch. Maybe someone would be able to help him there. Commit him - best to an asylum."
Lehmann failed to see the funny side of Wiese's comments, however, and is suing for £17,500 for having his "personal rights violated".
A court in Munich will hear the case later this week.