"You have to be honest and say that is very difficult. It's not just the difference in points, it's also the Chelsea philosophy of playing," he told British media about his side's title chances.
"They score one goal and they win. They defend a lot. They defend well ... They don't care what people say, what people think. They just want to win. And, because of that, I don't see them losing many points."
United have narrowed the gap to fourth-placed Arsenal to four points thanks to three straight wins, and Mourinho was reluctant to predict where his side might finish.
"Let's see where we finish. I don't want to say fourth, because I think we can do better than fourth," he added.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
"But we have the risk of not finishing fourth because the competition is so hard and the top teams have the same ambitions as we have. So I don't want to say fourth or third or fifth."
United host 18th placed Sunderland on Monday.
(Reporting by Shravanth Vijayakumar in Bengaluru; Editing by Peter Rutherford)
