Sportsmail understands that Mourinho’s trip to Hampden Park last month, when he watched Scotland’s 1-0 defeat by Costa Rica, had initially been scheduled as a scouting mission.
Tierney had been recommended by United scouts and Mourinho was hoping to check on the left back, until the 20-year-old withdrew from the squad.
United scouts will be at Hampden Park for Celtic’s clash with Rangers on Sunday afternoon
INSIGHT – CHRIS SUTTON
One thing that stands out straight away with Kieran Tierney is his personality. He’s only 20, but has already captained Scotland.
He’s level-headed, hard-working and very likeable. And he’s a grafter, someone who works hard for the team.
It doesn’t surprise me Manchester United are after him. He’s a Jose Mourinho-type player.
He really caught the eye in Europe last season and, amazingly, only Sporting Lisbon goalkeeper Rui Patricio has played more minutes in any top division across the world this season.
He’s extremely solid in defence, makes good decisions and can play anywhere across the backline.
He can improve. His final ball could be better and his attacking contribution is not as outstanding as it was last season.
That said, he is a top young talent with the mentality to make those improvements and United would be lucky to have him.
United midfielder Scott McTominay played in that friendly, but Mourinho’s travel arrangements were finalised before the 21-year-old had pledged allegiance to Scotland.
Mourinho caught up with former Scotland boss Craig Brown, to whom he became close while studying for his UEFA Pro Licence north of the border, and asked about Tierney’s development.
United are expected to have a scout at Hampden Park on Sunday when Celtic face Rangers in the Scottish Cup semi-finals.
Mourinho has made it clear to executive vice-chairman Ed Woodward that a new left back should be seen as a priority given persistent problems with Luke Shaw.
Tierney is raw and, while versatile, he may be considered more of a long-term prospect.
Mourinho, meanwhile, ruled out signing another striker on Friday as competition for Romelu Lukaku.
The Belgian has missed only two games all season, while Anthony Martial and Marcus Rashford have been on the periphery since Alexis Sanchez’s arrival.
Rashford has made more appearances than anyone under Mourinho, although many of those have come from the bench.
Tierney has been recommended to Jose Mourinho by United scouts in recent months
‘It would be amazing economically for the football clubs to have only 11 players and play with the same 11 — no injuries, suspension, fatigue,’ Mourinho said.
‘But it is not possible. At every club and every level, you need more than 11 players and only 11 can start, it is as simple as that, there’s nothing I can do.
‘Marcus is the player with more appearances since I arrived at the club, because I decide that way.’