Atletico Madrid may be without key midfielder Koke for over a month after he suffered a torn hamstring in his side's 4-0 thrashing of Real Madrid Saturday.
"The player suffered a torn hamstring in the back part of his left leg," the defending La Liga champions said in a statement, reports Xinhua.
The 23-year-old Koke is set to miss the Champions League last-16 first-leg against Bayer Leverkusen this month and possibly the league clashes against Sevilla and Valencia.
The 4-0 victory pulled Atletico within four points of their capital neighbours and level with Barcelona, who play their game-in-hand Sunday away at Athletic Bilbao.