Herrera is one of a number of players who are on the final year of their deals, but the Spaniard hasn't started many games this season.
United face the Serie A champions on Wednesday and Mourinho can not be certain whether the Belgium striker will be ready to travel.
After a nervy win over Everton last weekend, Manchester United came up with yet another last-gasp victory over Bournemouth this afternoon.
Manchester United won consecutive Premier League matches since September and are now with 10 points from possible 12 in the past four league matches, beating Bournemouth 2-1 after another comeback.
Rashford has now scored two goals and has made one assist in eight appearances in the Premier League this season.
United started the second half with much more intensity with Paul Pogba making more forward runs into the opposition defence.
Epitomised by his block in front of Marcus Rashford in the second half, the Dutch worldwide stuck to his task well all afternoon, and couldn't quite intercept Pogba's cross to Rashford for the decisive goal of the game.
"We were defensively very bad". Bad. I thought people watching this game wouldn't believe how hard we'd trained this week.
"The reason (for his omission) is he is playing lots of matches and lately, not just not scoring goals, but also not showing lots of confidence", said Mourinho.
Mourinho will know, however, there must be much more to come if they are to revive their challenge for a top four place.
"When the team is really defensive and you are more defensive-minded and you base your work on that, then the team doesn't develop enough in other areas but manages to get some results based on that", he said. "Some of our attacking movement was second to none". The lead can be overturned against teams like Newcastle and Bournemouth, but big games are coming thick and fast and United need to be ready for them from the first minute.
Following a Champions League game against Juventus, Manchester United have the small matter of the Manchester derby as their next fixture in the Premier League.
"I wouldn't say it's up", Burley said of Lukaku's United career.
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