Mourinho said Young was always in his plans, per Stuart Mathieson of the Manchester Evening News: "(There were) suggestions, but not from me. I always thought that he was not to leave, he wants to stay."
Young was a fringe player during Mourinho's first season in 2016/17, but he has become valuable again during the early portion of this campaign.
A report in August from Samuel Luckhurst of the Manchester Evening News revealed Young would stay beyond the end of the summer transfer window because United's asking price had "put off potential buyers."
United's squad has been short at the left-back position, particularly while Luke Shaw has struggled with injuries. Mourinho recently called on Shaw to continue working and improving as he bids to finally become a fixture in the Red Devils' starting XI, per Sky Sports: "Luke had surgery and a long time without playing football. I cannot expect him to be back and to be strong, strong, strong in his work."
Shaw's absence increased the pressure on players such as Daley Blind and Matteo Darmian to play out of position at left-back. It's a problem Young has solved thanks to better end product than either Blind or Darmian were providing.
Young supplied an assist during United's 3-0 win over FC Basel in the UEFA Champions league earlier this month. It was the signature moment of a standout performance from the veteran, showcasing his ability to also handle defensive responsibilities, as numbers from Squawka Football showed:
Young has since added legitimate defence-stretching pace on the break and regular teasing deliveries from the left flank. Continued strong performances may even encourage Mourinho and United to extend the player's stay at Old Trafford.
Mathieson noted, "Young signed a three-year deal in 2015 and the club has an extra year's option to keep him under the agreement."
United don't plan to discuss any extension until the new year, but Young is keen to stay on, per Luke Edwards of the Daily Telegraph.
Mathieson's report also detailed how Young has played both full-back positions for the Reds. His versatility and experience are proving invaluable for Mourinho's title-chasers.