Ruud van Nistelrooy and Cristiano Ronaldo were involved in a furious bust-up at United's training ground yesterday.
The pair clashed at the club's Carrington complex during training and were immediately summoned to boss Sir Alex Ferguson's office to explain themselves.
It is understood the row related to Ronaldo's sending-off in United's 3-1 derby defeat to Manchester City at the weekend.
Van Nistelrooy is understood to have been furious with Ronaldo for his lunge on Andy Cole, which left United with 10 men.
Sources claim van Nistelrooy accused Ronaldo of showing a "selfish" attitude in getting sent off.
There has been growing discontent among senior United players at Ronaldo's attitude of late, many feeling the 20-year-old has become too self-indulgent.