Wayne Rooney questioned Manchester City's decision to allow Kyle Walker to leave the club last month after watching Pep Guardiola's men slip to a 3-2 defeat by Real Madrid on Tuesday night.
Club captain Walker informed City that he wanted to explore opportunities elsewhere in the early stages of last month's transfer window, and subsequently joined Italian giants AC Milan on loan.
But Rooney is adamant that City should have put up more of a fight to keep the experienced defender.
City led twice at the Etihad on Tuesday, but collapsed in the closing stages as late goals from Brahim Diaz and Jude Bellingham handed Madrid the advantage in their Champions League play-off tie.