The South African played no part in Everton’s 3-1 pre-season friendly win at Hearts on Sunday and is now being assessed at the club’s Finch Farm training centre.
Everton are still waiting to discover the full extent of the injury but manager Roberto Martinez admits he does not expect Pienaar to be available for Watford’s visit to Goodison Park on August 8.
“Obviously it was a really sad moment for all of us when Stevie picked up that injury in training,” he told evertontv.
“He’s gone back now to be fully assessed by our medical team at Finch Farm and we’ll get the results very soon.
“But it does seem it’s a problem with his hamstring and clearly he’s going to miss the start of the season, which is a real shame because he worked with fantastic results and fantastic signs over the first three weeks of pre-season.
“We were very, very down about that situation and we hope to get Stevie back as soon as we can.”