Lukaku returned to training with United on Friday ahead of their clash with rivals City.
The Belgium frontman missed their last two games – comeback victories against Bournemouth and Juventus – through injury.
But speaking in his pre-match press conference this morning, Mourinho said: “Lukaku is training with the team this morning.
“If the answer is positive – and we have of course one more session tomorrow – he is ready.
“In this moment, I can only say that he trains with the team.”
But some of the Old Trafford faithful are desperate for the forward NOT to start against City.
“Please don’t play Lukaku,” one fan wrote on Twitter.
Another said: “Hope Lukaku won’t play, his too slow for such a great game.”
“If I hear lukaku is starting I will lose it,” a third fan wrote.
A fourth supporter commented: “Regardless of Lukaku’s fitness, no way should he start on Sunday.”
“@RomeluLukaku9 should never play for man utd again and should be sold in January as he is so s**t,” a fifth fan wrote.
Meanwhile, midfielder Paul Pogba is a doubt for Sunday’s showdown after missing training on Friday morning.
The 25-year-old played the full 90 minutes against former club Juventus on Wednesday as Jose Mourinho’s men came back to secure a stunning 2-1 win in the Champions League.
However, Pogba was conspicuous by his absence from United’s training session on Friday – an unneeded injury scare ahead of Sunday’s derby at the Etihad Stadium, where he netted a brace in April’s 3-2 victory.
The Red Devils will be hoping that what is being described as a “little injury” will not keep the World Cup winner out against City.