Manquillo out with fractured vertebra
The Atletico right back came down awkwardly on his neck after a midair collision with Cristiano Ronaldo in the 44th minute of Tuesday's 2-0 loss to Real Madrid.
Manquillo was taken to hospital where he will remain until Thursday after fracturing the C7 right vertebra - in the neck region - which was not displaced.
The 19-year-old Manquillo, who has been used sparingly this season, was able to leave the field under his own power but never returned. Ronaldo was shown a yellow card on the play.
Manquillo's absence grows Atletico's injury list with goalkeeper Thibaut Courtois, back Felipe Luis and striker David Villa also sidelined.
The club did receive some positive news on Wednesday, when the Spanish league's disciplinary committee upheld Atletico's appeal against a red card given to goalkeeper Daniel Aranzubia in Saturday's 2-0 loss at Almeria.
Aranzubia avoids a ban and will be available for the next league game against Valladolid on Saturday.
Atletico, which has lost three straight games after previously losing just once all season, is tied with Barcelona and Real Madrid atop the league standings.