Manchester United are keeping tabs on Eliaquim Mangala, the Porto central defender who came close to joining Manchester City for £37m in the January transfer window.
David Moyes sent scouts to watch the 23-year-old in Sunday's game between Porto and Gil Vincente, according to reports in Portugal.
City are expected to return for the player in the summer and, with Paris St-Germain also interested in signing him, United face a battle to secure his signature if he does decide to move.
Moyes is hoping to overhaul United's squad in the close season, with the manager being handed a mandate by the club to bring in at least four major signings.