That’s according to John Hartson.
Jose Mourinho has splashed the cash this summer, brining Romelu Lukaku, Nemanja Matic and Victor Lindelof to Old Trafford.
And with doubts hanging over Luke Shaw’s United future, pundit Hartson says Scotland international Tierney tops Mourinho’s wish list.
“I’m hearing he’s Manchester United’s number one target,” he told the BBC’s Scottish Football Podcast.
“But why on earth would Brendan Rodgers sell Tierney right now?
“If I’m Brendan Rodgers, I’m saying to [Jose] Mourinho, ‘Let him have another season here, keep your £25-30m for now, he’s got Champions League football, he loves playing for Celtic’.
“If Kieran Tierney has another season, if he’s outstanding in the Champions League, playing really well against the top teams, in the spotlight, all over the world being looked at by the top clubs.
“I’m hearing he’s Manchester United’s number one target”
“All of a sudden, next year or the year after, you’re getting £45-50m for him.
“That’s how inflated these prices are now in terms of everybody paying absolutely massive odds, because the transfer have gone absolutely ridiculous. It’s absurd.
“I have every confidence in Kieran Tierney to become a top, top player. He is a great player now, but he can only get better.”
Tierney, 20, has made 85 appearances for Celtic since his debut in 2015.