Toni Kroos should not have been allowed to leave for Real Madrid, according to Bayern Munich sporting director Matthias Sammer.
The Germany international made his Bayern debut in 2007 and went on to lift nine major honours across a seven-year spell, including the treble of Bundesliga, DFB-Pokal and Champions League in 2012-13.
He left at the end of the following season, however, and has since gone on to further glory with Madrid, securing historic back-to-back European crowns as well as last term's LaLiga title.
Alongside Luka Modric, Kroos has formed arguably the finest midfield pairing in world football and Sammer feels Bayern are living to regret their decision.
"It was a mistake to let Toni go, his departure was bad, very bad," he told Bild.
"[But] for the personal evolution and recognition of Toni, the transfer was certainly a perfect decision."