United could fall 15 points behind Premier League leaders Manchester City if their rivals win at Newcastle on Wednesday following a 2-2 draw with Burnley at Old Trafford.
It might even have been worse for Mourinho's men had Jesse Lingard not come off the bench to score twice, including a stoppage-time equaliser, after the visitors had opened up a 2-0 lead.
Responding to a question that such a gap might be unacceptable for a club of United's stature, Mourinho said in his press conference: "One thing is a big club and another thing is a big football team. They are two different things.
"We are in the second year of trying to rebuild a football team that is not one of the best teams in the world. Manchester City buy full-backs for the price of strikers. When you speak about big football clubs, you are speaking about the history of the club."
"The big historical clubs, they are normally punished in the market because of their history."
Mourinho was then asked if he intended to spend big in the January transfer window but left the room before answering.
Prior to that, he bemoaned his side's defending. United conceded within three minutes to Ashley Barnes after failing to clear a free-kick into the box.
After being unhappy with the goals conceded in Saturday's 2-2 draw with Leicester, he said such defending should be described with a word beginning with S. He left the actual word to the imagination of those present.
He said: "Another 's' goal. I don't need to say the other letters of the word. That is what we are having now, lots of 's' goals against so that has an impact on the game."Source: Sky Sports