Kirk sustained a calf injury during Sunday's practice and is not practicing Monday, Nick Wagoner of ESPN.com reports.
Kirk walked off the field with trainers Sunday. Soft-tissue injuries are common during training camp, as players are ramping up to full speed and in pads after a long offseason. The severity of Kirk's calf strain isn't known, but consider him day-to-day. Kirk signed with the Niners in March and is expected to operate as the primary slot receiver alongside Mike Evans and Ricky Pearsall this season.