Lukaku, who has four goals for United this season and has failed to score for his club since mid-September, missed last Saturday's trip to Bournemouth with a muscle injury.
"(Lukaku is) injured for tomorrow and we don't know for Sunday," Mourinho told a news conference in Turin ahead of Wednesday's Champions League clash.
The match in Turin on Wednesday pits Juve's Cristiano Ronaldo against the team he played for from 2003-09.
Serie A leaders Juventus are top of Group H on nine points, with United second on four.
Mourinho said his team had to learn lessons from last month's 1-0 home defeat by the Italian champions and try and start on the front foot.
"We work every day... but the reality is that normally we don’t start matches well, but if we don't start well and we end well, that's fine," he added.
"It's good fun for the fans, but football is 90 minutes and especially when your opponent is strong, if you give 45 minute advantage, or even 20 or even 10, you have a mountain to climb.
"The good thing is that lately, apart from the (last) Juventus match, we were strong enough to climb the mountain and change the match in the last minutes. When you play against a super team, like tomorrow, if you give an advantage away, it's difficult to climb the mountain."
(Reporting by Alan Baldwin and Simon Jennings, editing by Pritha Sarkar)
