Twenty-year-old Mount - a former Purbrook Park pupil and one-time season-ticket holder at Pompey - was making his senior debut for Chelsea as the Londoners' kicked off their Premier League campaign at Old Trafford.
Lineker tweeted: "This is definitely Man Utd's best performance with Mourinho at Old Trafford". "It would have been incredible for him to be here today, but he's not, and I dedicate that goal to him". For them and for me, as I said you couldn't write it.
"People talk about the lad at Liverpool [Alexander-Arnold] but he can't defend like Wan Bissaka, whose defending and positioning is very good".
Chelsea began their Premier League season with a 4-0 thrashing at Manchester United on Sunday, with a Marcus Rashford double as well as goals from Anthony Martial and Daniel James condemning Frank Lampard's side to a comprehensive, opening-day defeat.
He was one of two big-money defensive signings made by the Red Devils along with £80m Harry Maguire.
"There's always a question about him going forward, he can learn that, he will get better and better".
Lampard himself was actually playing for West Ham the last time that Chelsea were stationed in the drop zone, not completing his move to Stamford Bridge until July 2001.
"If you're playing next to him as a centre-half, you're saying to yourself "there's my get out of jail card" because he wants to make those tackles and wants to protect that right hand side". I thrive on that.