But that hasn't stopped David Moyes trusting the Belgian midfielder with the club's present. It's a brave decision from Moyes, who hasn't got time to let players learn on the job. Each negative result he oversees during this tricky transitional phase will increase the pressure on him just a little bit.
After all, Moyes doesn't have to pick Januzaj.
Against Sunderland, instead of handing the former Anderlecht youngster his full Premier League debut, he could have chosen Antonio Valencia, Danny Welbeck, Shinji Kagawa, Ashley Young or Wilfried Zaha.
They are all more experienced and would perhaps be considered safer hands.
But Moyes rates Januzaj highly.
He watched United struggle to break down West Brom at Old Trafford last week, and his answer at half-time was to throw on Januzaj in place of Kagawa.
It backfired slightly that day after Morgan Amalfitano ran off him to give West Brom the lead, and that's an area Januzaj will have to improve.
At the Stadium of Light, he lost a battle with Ondrej Celustka in the build-up to Sunderland's opener, although Phil Jones and Nemanja Vidic will shoulder more blame.
But that side of Januzaj's game will develop as he gets older and stronger.
Going forward, however, he was one of United's brightest lights. The fact that he overshadowed Robin van Persie and Wayne Rooney is quite a compliment, but not undeserved.
He offers a different option to United's other wide players. He drifts past full-backs, relying on exquisite balance and subtle body movements.
He hasn't got Valencia's strength and power or Nani's blistering pace, but it doesn't stop him being effective and dangerous.
With United a goal down and facing a third straight league defeat for the first time since 2001, Januzaj wandered inside. He picked up the ball with his back to goal, spun and rolled it out to Rooney. Instead of admiring his pass, he carried on and was rewarded with the chance to calmly side-foot his team level.
But the most impressive part of the passage of play was his reaction to the ball hitting the net. Plenty of teenagers would have been blowing kisses to the crowd after scoring on their full debut.
Not Januzaj. He ran to get the ball out of the net. If the pass and finish showed his technical ability, the reaction was proof of his temperament.
But six minutes later he was celebrating, after crashing a perfectly-executed left-foot volley past Keiren Westwood. United went from 1-0 down to 2-1 up in the blink of an eye thanks to a boy who, before this season, had been an unused substitute in the first team just once.
Januzaj arrived at the Stadium of Light as a promising prospect. He left a fully-fledged member of United's first-team squad. United fans knew that, with him in the ranks, they could look forward to a bright future.
He's making the present looking a little rosier, too.