Vidic was hurt in training yesterday and national team spokesman Aleksandar Boskovic confirmed today the centre-half will not be available for the remainder of the Serbo-Montenegrins' matches in Germany.
The 24-year-old now faces a race to be fit for the start of United's Premiership season in August.
Boskovic said: "Our national team will have just 21 players for the rest of our games in Germany as Nemanja Vidic is sidelined because of a serious knee injury.
"During yesterday's practice he went to block a shot but, unfortunately, he rotated his knee too much.
"His yell was horrendous and we all knew immediately something bad had happened.
"He was taken to hospital and it was found he had damaged his medial collateral ligament.
"He must rest for several weeks, according to our national team doctor Ilija Asanin.
"Nemanja will remain with us here in Germany. He is in stable condition and is psychologically good, as much as a youngster who dreamed about the World Cup can be after suffering such a thing."