It says on the list, adductor, which is groin.
I agree that stop-start can cause further injuries but it's a hard thing to call, if he and the staff felt he was fit to then expect an injury in another area of the leg.
@Hustini
Use your slightly weird but magical powers to heal Chiellini's left calf, just touch the screen and repeat 'forza Giorgione' several times per day.