The Argentine forward is yet to agree a new contract at Barcelona, but the Red Devils acknowledge that they would not be his first choice if a move was made.
Manchester United have conceded that arch-rivals City would be favourites to sign Lionel Messi if he were to ever leave Barcelona, reports The Sun.
The Argentine forward is yet to agree an extension at Camp Nou to a contract that is due to expire in 2019.
That has sparked rumours of a move elsewhere, although all parties in Catalunya have sought to play down rumours that the five-time World Player of the Year could be allowed to move on.
In the unlikely event that he does become available, then clubs in the Premier League would be expected to lead the hunt for his services as they boast many of the sides able to afford a deal.
United, though, are said to have already accepted that the riches at City – and the presence of former Barca boss Pep Guardiola at the Etihad Stadium – would likely work against them.