McGuinness, who left the youth set up last February, oversaw 86 youngsters break into the first team during his time at Old Trafford.
Borthwick-Jackson made his debut against West Bromwich Albion last November and has gone on to make 10 appearances for the senior side, impressing many in the process.
The 50-year-old insisted that Borthwick-Jackson will provide stern competition for Shaw – who is set to make his comeback on Saturday after 10 months out – next season.
“He’s another local lad who has just been a steady-Eddie and kept improving,” he said.
“He’s done very well when he’s been in the first team, looked very calm and with no nerves. He’s been strong defensively in winning balls in the air and not being beaten but there have also been some fantastic crosses going forwards.
“He’ll have a battle on his hands when Luke Shaw is back because he’s a fantastic young player as well but to have two young players like that bodes well for the future.”
In Shaw and Borthwick-Jackson, Mourinho has two of the brightest young left-backs in the country at his disposal. One silver lining of Shaw’s horrific leg break back in September was the emergence of Borthwick-Jackson as a result.
The 19-year-old insists that he never feels nervous before a game, and such an attitude is visible in the way he cooly conducts himself on the pitch, nonchalantly taking the ball in his stride and playing positively and assuredly.
His superb crossing ability, displayed on several occasions last season, arguably eclipses that of Shaw. And as the latter continues to work towards full fitness, he’ll have his work cut out in attempting to stay above Borthwick-Jackson in the left-back pecking order.