David de Gea mystery injury swells uncertainty over Manchester United future amid Real Madrid talk

David de Gea's future at Manchester United continues to be clouded in uncertainty after the goalkeeper was ruled out of the club's second pre-season match of their US tour with an unknown injury.

De Gea was absent for the win over Club America with a "minor injury" and will miss the game against San Jose Earthquakes on 22 July, but manager Louis van Gaal has refused to disclose the nature of the problem. Antonio Valencia will also miss the game with an unidentified injury.

Speaking prior to the International Champions Cup match, Van Gaal told reporters, according to the club's official website: "They are minor injuries so they have visited the hospital this morning. It is confirmed [De Gea and Valencia] cannot play against San Jose, I think, but we have to wait."

The Dutch coach added to MUTV: "It's the same line-up in the first half and the second half, only Sam Johnstone shall play 90 minutes because we don't take any risks with the other two goalkeepers."

When questioned by assembled journalists over the specifics regarding De Gea's injury, Van Gaal declined to respond, calling into question the true reason behind his peripheral pre-season role.

Concerns over De Gea's future this summer were seemingly quashed when the Spanish international was included in the club's squad for the four-match pre-season tour; however his continued omission from the United team fuels speculation over a potential move to Real Madrid.

Real have signed Kiko Casilla from Espanyol as cover for the departed Iker Casillas and manager Rafael Benitez has denied the club are interested in signing De Gea, who has one year remaining on his United contract.

Sam Johnstone, who has never made a professional appearance for United, will make his second start of the tour to strengthen his position ahead of Anders Lindegaard. The battle for reserve keeper is the first of what is expected to be several selection dilemmas this term for Van Gaal.

While new signings Memphis Depay, Morgan Schneiderlin and Bastian Schweinsteiger will compete for roles in midfield, Matteo Darmian will hope to feature in a United defence that suffered from constant injury last term.

But Van Gaal says despite arriving with a decent reputation from Torino, Darmian will start the campaign as second choice behind Valencia – who occupied the right-back position for much of last year.

"We have brought Darmian for the second position behind Valencia and now already because of that injury [Paddy] McNair has to play there," he said according to ESPN.