The Mail on Sunday says Keane spent Saturday at the V Festival in Chelmsford with his wife Claudine as Spurs won 2-1 at Stoke.
The striker swapped the plastic that caused him and Tottenham Hotspur so much pain in midweek for the more comfortable feel of grass beneath his feet.
He suffered a knee injury during Tottenham's 3-2 Champions League qualifying defeat against Young Boys in Berne last week on an artificial surface. Now he is struggling to be fit for the return at White Hart Lane on Wednesday, along with Jermain Defoe, Luka Modric and Roman Pavlyuchenko.
It is believed that Keane can run, twist and turn but is in pain when it comes to kicking the ball.
Spurs were comfortable with him attending the rock event, which is being headlined by Kings of Leon and Kasabian this weekend.
"Robbie was in for treatment this morning on a knee injury that kept him out of today's game. After completing his club requirements, he was free to spend his time as he wished," said a Tottenham spokesman.