Real Madrid's Dutch international Wesley Sneijder suffered serious knee damage during a heavy tackle by Arsenal's Abou Diaby in Sunday's match in London, the Spanish club said.
Midfielder Sneijder was stretchered off in the second half of the Emirates Cup match which Arsenal won 1-0.
'Initial tests show serious damage to his knee ligaments, but the player will undergo complementary exams including an MRI on Monday to determine the full extent of the injury,' Real said on their website.
The 24-year-old was about to start his second season with Real after joining the club from Ajax last year.
His absence from the squad for the start of the season would be a hard blow for the Spanish champions.
But Real Madrid have set their sights on another Dutch midfielder, Rafael van der Vaart of Hamburg, and have made a nine million-euros (14 million-dollars) swoop for the player.