The 25-year-old is heavily being linked with an exit after failing to hold down a regular role under Luis Enrique as his former boss looks to lure him to the Etihad.
Guardiola, who previous worked with the defender at Barca, was the one who handed him his debut six years ago at the age of 19.
However, City will face tough competition from United as the club looks to add more defensive reinforcements into their ranks during the summer window.
Tottenham Hotspur are another club keen to lure the centre-back to the Premier League as they look to build from their sensational campaign.
The Spain international has only started three La Liga matches while featuring another eight times from the bench this term, so far managing to score one goal while providing another assist.