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 the Bundesliga title, DFB-Pokal Cup and the Champions League in 2012-13.
He left at the end of the following season, however, and has since gone on to achieve further glory with Madrid, securing historic back-to-back European crowns as well as last term's La Liga 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 [Kroos] 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."
However, it hasn't affected Bayern too much as they trashed Besiktas 5-0 in the first leg of their Champions League tie to look an almost sure bet to progress to the quarter-finals.