"We have to wait until the last moment," Mourinho told a news conference on Friday.
"I don't want to lie to you and say he doesn’t play and then plays, so with Lukaku we have to wait."
Mourinho confirmed that Anthony Martial is available for selection after a knee injury ruled out the French winger for United's win against Watford on the final day of the season.
United head into the Wembley final after finishing second in the Premier league, their best season in the top flight for five years.
It is also an opportunity for United to claim their third major trophy under Mourinho, having won the Europa League and League Cup last season.
But the Portuguese manager chose to play down the significance of the match, saying he will not regard United's season as a failure even if they lose against Chelsea.
"You can analyse the way you want, and I can analyse the way I want," Mourinho added.
"I know what my players did. I know the effort they gave. The positive things, the negative things. I'm not going to change my opinion on one match, not at all."
(Reporting by Hardik Vyas in Bengaluru; Editing by David Goodman)
