The Red Devils may have more luck in attempting to sign the Senegal international in the summer.
The Mail reports that the Red Devils hoped to complete a deal with the Saints this week. However, the south coast club have stood firm and told Louis van Gaal that the Senegal international is not available during this window.
While Southampton's stance may result in United pursuing other targets this month, a move in the summer could well be on the cards, as the Mail suggests that the Saints will listen to offers at the end of the season.
Despite enjoying a blistering first campaign at St Mary's - during which he helped the club defy the pre-season predictions that their summer exodus would ultimately result in relegation to finish 7th, and also broke the record for the fastest ever hat-trick in the Premier League - Mane's relationship with the club has seemingly turned sour recently.
He was publicly criticised by Ronald Koeman after he turned up late for a team meeting ahead of their game with Norwich City (via Mail). And while he has scored three times and managed six assists, it is probably fair to say that his performances have not quite been up to the same standard as last season.
And with United clearly still interested, it appears that Mane's time as a Southampton player is set to come to an end in the coming months.