The injury forced the striker to depart early from Saturday's 4-1 win against Sunderland at the Lane.
It's a blow for Robbie having returned to the line-up after missing last week's draw at Chelsea with an ankle problem.
Boss Glenn Hoddle said the Irish star simply tweaked the knee as he attempted to keep the ball in near the touchline. He came off with only 20 minutes on the clock.
"It's been an enjoyable afternoon but the only down side is Robbie," sighed Glenn. "He picked up a slight ligament strain.
"He'll have a scan and we'll see the degree of it but any ligament strain is not good.
"The fact we whipped him off sooner rather than later might have protected him a little. Hopefully it won't be too bad."
Robbie Keane will undergo scans to determine the extent of a ligament strain in his knee.