The 20-year-old Cardiff Blues wing made his British & Irish Lions debut against the Free State Cheetahs in Bloemfontein last weekend after spending 10 days at home overcoming his thigh problems.
But he aggravated the injury at the end of the training session on Monday and has been withdrawn from the Lions replacements as a precaution.
"He was examined by the medical team this morning and was comfortable. However, in order to enable an accurate diagnosis he is currently having a scan," said a Lions spokesman.
"As a preliminary measure he will be unavailable for Wednesday's game and James Hook will take his place on the bench."