Januzaj was loaned to the Wearsiders for the season but hasn't given up on his dream of making the grade at Manchester United – despite struggling to make an impact with the relegation-threatened club, scoring one goal in 24 appearances in all competitions, and being booed by Mackems fans in a recent game.
Mourinho said: “Yes. I have decided. Can I say what it is? Not to you!"
It seems unlikely Januzaj, 22, will be part of Mourinho's future plans however, with young trio Marcus Rashford, Anthony Martial and Jesse Lingard all ahead of him in the pecking order at United.
Meanwhile Mourinho has sympathised with David Moyes' problems at Sunderland.
United will be looking to pile on the problems for Moyes when they visit the Stadium of Light on Sunday but, having been sacked himself by Chelsea last season, understands the pressure one of his predecessors at Old Trafford is under.
Jose Mourinho has made a decision on Adnan Januzaj's future
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He said: "“I have sympathy for any manager that is fighting hard to reach his objectives, and he's in trouble to do it.
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“In the whole season that can happen with so many of us.
“At the end of it there is one team that is going to be champions and three teams that will be relegated.
“But everyone is a good professional, gives everything and fights all season.
“Some of us are sacked in the middle of the season so it's difficult for everyone and of course I have sympathy. That's normal."
The United job proved too much for Moyes who was sacked 10 months into a six-year contract three years ago this month.
Adnan Januzaj is currently on loan at Sunderland
Asked what it is like being Untie dmanager, Mourinho said: “You'd have to ask him his feelings. For me it's an honour, a pleasure and the continuity of my career.
“I've worked at some of the biggest clubs in the world and I've been lucky to have the experience of doing that in different countries which makes me even richer from a cultural point of view in terms of football experiences. I'm enjoying it.
Ashley Young has joined Unityed's injury list which also includes Wayne Rooeny, Juan Mata, Chris Smalling and Phil Jones.
Mourinho added: "Rooney is not available, Young is injured and there's no more news because you already know that with Mata, Smalling and Jones we're speaking about long-term injuries.”