The 23-year-old defender had missed Cote d'Ivoire's failed 2018 Fifa World Cup qualification bid to Morocco last Saturday due to suspension.
And Mourinho confirmed that the former Villarreal defender who scored his first Manchester United goal in their 4-0 defeat over Swansea City on August 19, was omitted from Saturday’s English Premier League fixture against Rafa Benitez’s men due to an injury.
"Defender Bailly was a notable absentee from the XI to face the Magpies at Old Trafford in the first game after the international break, with Chris Smalling and Victor Lindelof named as the centre-back pairing,” the statement reads on the club website.
“Mourinho disclosed the reason for Bailly’s absence in his pre-match interview with BT Sport and also confirmed Henrikh Mkhitaryan, another name missing from the matchday squad, was not selected for tactical reasons.”
Bailly had suffered a groin strain in Cote d’Ivoire’s World Cup qualifying game against Mali on October 6 and was sidelined for three weeks before he returned in Manchester United's 1-0 win over Tottenham Hotspur on October 28.