Defoe sat out the 2-1 FA Cup fourth-round loss to Leeds at Elland Road due to a problem with his pelvic muscle.
But Villas-Boas said Defoe had undergone scans and had a plasma injection to allow him to be fit for the Premier League trip to Norwich on Wednesday night.
When asked about the extent of Defoe's injury, Villas-Boas said: "He felt some tightness. It was a Grade 1 injury to his obturator muscle which was treated immediately.
"We acknowledged that resting him for Leeds was almost compulsory so we let him out of the game and he has made an excellent recovery so he is back.
"In muscle injuries there is always a risk of it showing up again, but the medical department feel positive about it and the player is positive and he knows his body well.
"We believe a lot in growth factor treatment and plasma platelet treatments. It is a technique that we have been using since the beginning of the season.
"It is not common but does happen from club to club. The player adhered to the treatment very, very well so we can move forward.
"The day he felt the injury was Friday and he had the injection thing. Now he is available for selection. That is the remarkable thing."