Premier League clubs will be on alert after Ricky van Wolfswinkel's contract extension talks with Sporting Lisbon suffered a hitch.
The Dutch striker has enjoyed a fine first season in Portugal since his £4.5million switch from Utrecht last summer.
The 23-year-old fired 20 goals in all competitions, taking Sporting all the way to the semi-finals of the Europa League.
That has prompted talk of a new deal for Van Wolfswinkel in order to increase the release clause in his contract, but those discussions have stalled.
Both Manchester United and Manchester City are thought to be tracking the Holland international and the lack of progress could open the door for a bid.
The player's agent Louis Laros admits there could be time for a late move as no deal has yet been done.
"We are waiting for Sporting," Laros told Portuguese newspaper Record.
"There is still no progress in increasing the value of the buy-out clause that the club want to include.
"I have heard rumours in the media about interest in the player from foreign clubs."
Manchester City know all about Van Wolfswinkel after he scored an important away goal at the Etihad Stadium to help eliminate Roberto Mancini's men from the Europa League in March.