The 17-year-old signed for United in January from Partizan Belgrade, but has been loaned back to the Serb side until next year.
Ljajic said: “The quality in England is very high because you are training with such great players but the main difference is it’s much quicker.
“I really enjoyed it and training with such top players can only help me improve.
“Coming over to England allowed me to see how things worked so I know what to expect when I come here permanently.
“And I hope that when I join in January I will be involved with the first team from the start.”
