Mane is set for a Bayern medical on Tuesday.
"Nothing against the club, it's a great club, but the league is nowhere near a challenge for Mane," the 75-capped Wales forward declared on talkSPORT.
"He'll get goals. They win 5-0 every week, only one team can win the league. It's not a challenge. He will get in his armchair, light a cigar up, he'll play for Bayern in third gear and he'll coast for two years and ruin the best two years of his life as a footballer.
"At the end of his career, if he said to you 'come and look at my trophy cabinet', the German league winners medal will be right at the back."