United will complete the signing of Sweden’s Victor Lindelof in the next 24 hours, leaving Smalling surplus to requirements.
United have agreed a fee of £31m with Benfica for Lindelof, who will travel to Manchester today to undergo a medical and thrash out a long-term deal.
Mourinho wants Lindelof to partner Eric Bailly in the heart of his United defence, with Phil Jones providing back-up.
He has won two Premier League titles and both domestic Cups in seven seasons at Old Trafford.
Smalling was also voted United ‘Players’ Player of the Year’ last season following a consistent campaign under former boss Louis van Gaal.