Rafael da Silva has left Manchester United's tour of the United States because of a groin injury, the club have confirmed.
The Brazilian right-back missed Saturday's 3-2 win over Roma and United do not believe he will be fit enough to feature in their remaining four preseason games in the U.S.
Rafael, who had figured as a substitute in the 7-0 win over LA Galaxy, missed chunks of last season with thigh and groin injuries.
His departure leaves new United manager Louis van Gaal short of proven defenders after the summer departures of Rio Ferdinand, Nemanja Vidic and Patrice Evra.
Van Gaal has switched to a 3-4-1-2 formation and, in Rafael's absence, wingers Antonio Valencia and Ashley Young were used at right wing-back against Roma.
Meanwhile, Juan Mata has praised manager Van Gaal for his "demanding" nature after the Dutchman criticised his new charges for conceding two late goals against the Italian side.
The Spaniard told national newspapers: "He is right to be demanding. As a player you want a coach like this, always demanding more and to play better and try to win the games earlier or in a different way. After the game, when we had given two goals away, he was not very happy, but I think his way is that of a great manager."
Mata became United's club record signing when he joined from Chelsea for 37.1 million pounds in January but, despite his price tag, he was used as a left and right winger, as well as a No. 10, by former manager David Moyes.
Van Gaal has installed him in a central role behind two strikers and Mata said it is his favourite position.
He added: "Things are changing and everyone is finding their new role in the team, but if I play behind the two strikers I feel comfortable.
"I have played on the left or right, but my best season was when I played in that position under Rafa Benitez at Chelsea."