The 20-year-old Argentine posted a picture on social media of him wearing Villa’s home kit with the name of his United team-mate Marcus Rashford on the back.
Rashford, who spent the second half of last season on loan in the Midlands, later responded by commenting “my brother” in response to the post, along with a love heart emoji.
Garnacho’s gesture is thought to have been intended as one of support to Rashford, who has been frozen out at Old Trafford under Ruben Amorim.
Yet being pictured in the shirt of a rival club has, understandably, not gone down well with United supporters.
Garnacho’s own future at the club is in doubt after Amorim told him he could find a new club this summer. The player’s post is only likely to reinforce the United boss’ viewpoint.
Villa are among the clubs keeping tabs on Garnacho’s situation, though the finances involved for a player valued by United at around £70million are thought to be beyond them at this point.
Unai Emery would also like to bring Rashford back to Villa Park but their failure to qualify for the Champions League has made the deal tricky.
Villa had an option to buy Rashford for £40m included in the loan deal but that clause will not be triggered.
Rashford is keen to leave United permanently and ideally would like to join Barcelona, having recently expressed his desire to play alongside teenage sensation Lamine Yamal.
But the Catalan club are close to signing Nico Williams from Athletic Bilbao, making a move for the older Rashford less likely.
Villa, who are having to tread carefully as they seek to comply with both the Premier League and Uefa’s financial rules, could re-emerge as an option for Rashford later in the window should he fail to secure a permanent move elsewhere.