Barcelona coach Pep Guardiola was admitted to hospital overnight for treatment on acute lower back pain, the La Liga club said today.
The 40-year-old former Barca and Spain midfielder had to cancel a news conference on Tuesday because of the complaint and looked in pain during the champions' 1-0 win at Valencia on Wednesday.
"Pep is not feeling great and he got through the match as best he could," Guardiola's assistant, Tito Vilanova, said at a news conference after the game at the Mestalla.
"He is having a bad time of it and he doesn't even know where to put himself."
Guardiola's back problem comes at a crucial time for the Spanish champions.
They have away matches at Sevilla and Villarreal in coming weeks and will attempt to overturn a 2-1 deficit against English club Arsenal in their Champions League last-16, second leg at the Nou Camp on Tuesday.