Manchester United striker Romelu Lukaku to miss out UEFA Champions League clash with Juventus in Turin due to an injury that could also see him sit out derby against Manchester City, according to Mourinho.
Red Devils squad travel to Italy to face Group H leaders in Allianz Stadium on Wednesday night after coming from behind to beat Bournemouth 2-1 on Saturday.
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The Belguim international is struggling with for form in front of goal this season and he dropped for Manchester United last two matches in the Premier League encounter at Dean Court but the 25-year-old was not included in Mourinho squad to kick-start their push for the last 16.
The Portuguese coach fears the striker could also absent for the Manchester derby with a "muscle, tendon" issue after former Inter Milan boss confirmed Lukaku absent in Champion League tie via the club website.
Mourinho said: "Injured for tomorrow and we don't know for Sunday,"
"We have to wait for tomorrow, wait until Monday,” said the United manager at the time.
"He needs these couple of days. It was something in training. We don't know, muscle, tendon... we have to wait a little bit.”
"We work every day to try (and change that), but the reality is that normally we don't start matches well," Mourinho said.
Manchester United have needed late comebacks to win recent matches against Bournemouth and Newcastle, but Mourinho said the club cannot afford to fall too far behind against a team of Juventus' quality.
"We work every day to try but the reality is we don't start matches well," he said. "If you don't and we end well that's fine, it's even good fun for the fans.
"But football is 90 minutes and especially when the opponent is strong if you give 45, 20, even 10 minutes advantage, then you have a mountain to climb.
"The good thing is lately apart from Juve we were strong enough to climb the mountain, change results and win matches.
"When you play against a super team it's very difficult to climb the mountain then."