Torres had to be replaced by Arsenal star Cesc Fabregas in the 15th minute in this 2010 FIFA World Cup South Africa™ qualifier.
Spain team doctor Oscar Celada told Spanish television: "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."
UEFA EURO 2008 champions Spain lead Group 5 but only after David Villa struck in the closing stages.
Belgium took the lead from Wesley Sonck's seventh minute header, the first goal Spain keeper Iker Casillas has conceded since 14 June 2007.