The Red Devils continue to be heavily linked with a £38million move for Benfica's Swedish defender Victor Lindelof, and according to Mourinho, there will only be one new player coming to Old Trafford in January.
"If we buy a player we buy a player, we're not buying two, three or four," he told Sky Sports News HQ.
"I believe more in the summer market than in the winter window. More time, more choice, I think it's better. But let's see what happens."
It is also understood that Portuguese giants Benfica have agreed a £700,000 deal for Huachipato's teenage Chilean right-back Simon Ramirez to replace the outgoing Lindelof.