The 25-year-old suffered the injury during Friday’s NatWest T20 Blast game against Worcestershire and he is now expected to be out of action for some time.
It is a blow to both player and team.
Willey has recently established himself as a member of England’s rejuvenated one-day side and would be keen to back up those performances with his county.
Meanwhile, Northamptonshire are embarking on one of their busiest periods of the season and need all their key players to be fit.
A brief statement from the club said: “David sustained ankle ligament damage in our T20 at New Road.
“(An) MRI showed a low grade strain that will keep him sidelined until the back end off July”.