He will join countryman Nemanja Vidic at Old Trafford and wear the number 14 shirt while Ljajic will arrive back at United for the 2009/10 season.
Ferguson said: "We have been watching Tosic for over a year and in that time, he has become a regular in the Serbia national team.
"He's a winger and we like wide players here at Manchester United, so we are very hopeful that he will do well for us.
"He hasn't played since 10 December, however, so he will need some time to adjust to the speed of the game over here and get used to the training.
"But once he gets himself accustomed to the pace, then I am sure he will play a part for us this season. Ljajic is another good player with real quality and he will come over to Manchester in time for next season."
Tosic increases manager Sir Alex Ferguson's options on the left wing with Ryan Giggs and Nani already battling it out in that position although in recent weeks Ji-sung Park has also played there.
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