Januzaj, who recently returned to Old Trafford after ending his loan spell at Borussia Dortmund early, shares the same birthday as Real Madrid star Cristiano Ronaldo.
Young posted the picture above on his personal Instagram account on Friday afternoon and wrote: “Happy birthday bro.”
The Belgium international made his first appearance of the season for United when he came on as a late substitute during the 1-0 loss to Southampton at Old Trafford last month.
Januzaj will be hoping to feature when United return to Premier League action for their crunch clash with Chelsea at Stamford Bridge on Sunday afternoon.
The Red Devils head into the game looking to build on their much-improved performance during their 3-0 win over Stoke City at Old Trafford on Tuesday night.
United are currently five points adrift of the top four.
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