The 28-year-old Italian finished 15th in last week’s Akron tournament despite the injury, but conceded that it is still causing him problems.
“It’s very painful when I try to hit a ball, especially out of the rough and with the turf firm as it has been this week,” he told Sky Sports News.
“The cure is to just rest it. You can’t really rest it during the major weeks.
“You never know with these things. I am working on with it with my physiotherapist and I am still hoping it will be better tomorrow and I will be fine for Thursday.
Molinari’s best major performance came at last year’s US PGA event, where he finished tied for tenth.