Pienaar came off in the 24th minute at White Hart Lane after pulling up sharply and Redknapp confirmed after the match that he will go to a specialist to check the extent of the injury.
That could mean Pienaar misses the start of Tottenham's Premier League campaign against his former club Everton next weekend.
"Steven Pienaar picked up an injury today," Redknapp told ITV4. "He might have torn his groin. I think we're going to take him to a specialist and have a look at him. We'll see how it goes."
Meanwhile, Tottenham's unsettled midfielder Luka Modric received a surprisingly warm reception from his club's fans when he came on for Pienaar.
Despite declaring in pre-season that he wants to leave for Chelsea, the Croatian was cheered onto the pitch.
Mikel San Jose put Bilbao ahead soon after Modric's arrival but two goals in two minutes from Peter Crouch and Jermain Defoe early in the second half put Spurs in control.
Carlos Gurpegi upended Aaron Lennon in the box to give Spurs a chance to move further in front in the 70th minute, but Gorka Iraizoz saved Niko Kranjcar's penalty.
Source: AFP
Source: AFP