This was reported by Ian Rapoport of NFL.com:
Following an MRI, #Patriots G Jonathan Cooper was diagnosed with a plantar fascia strain, source said. Will be off his feet for a bit.
— Ian Rapoport (@RapSheet) July 31, 2016
Now, I know this sounds bad, especially with Cooper’s history of injuries, but this isn’t the end of the world for his future in New England right now. In fact, it sounds like he will be back for the start of the regular season, if not earlier than that.