The club tweeted: 'Following further investigation, Robin van Persie has a sprained knee which will keep him out for around four to six weeks.'
Moyes had earlier on Friday suggested that there could be bad news regarding the former Arsenal man.
'We're getting Robin re-scanned again today,' the Scot had said. 'We're just not quite conclusive on everything we've seen yet. We want to just check that out and be sure. He won't be available for (Saturday's) game (against West Ham). But we'll try to give you some update when we can on the outcome of it.