The striker lasted less than 16 minutes of his country's World Cup qualifier in Belgium last night before being replaced by Cesc Fabregas.
Early indications suggest the striker will be out for a minimum of 10 days, which would rule him out of Saturday's visit of Wigan Athletic and an emotional return to former club Atletico Madrid in the Champions League on Wednesday.
It would also make Torres a major doubt for the Premier League trip to Chelsea on Sunday week, a fixture from which he was also absent last season.
But Spain team doctor Oscar Celada said Torres may only be out for a week.
He said: "He has intense pain in the back part of the thigh.
"It is difficult to diagnose now exactly the extent, if it is a question of a strain or a small tear. If it is a strain then he could be out for about a week."