BUFFALO, N.Y. — The Buffalo Bills have placed running back Frank Gore on the non-football injury list due to what is considered a minor injury, the Associated Press reported.

The nature of Gore’s injury or how he was hurt remains unclear. Gore was placed on the list Monday, three days before the Bills open training camp in suburban Rochester, .

Buffalo signed the 36-year-old Gore to a one-year contract in free agency in March. He is the ’s active leader with 14,748 yards rushing and being counted upon to play a backup role behind LeSean McCoy.

The ’s transactions list also shows Buffalo placing tight end Tyler Kroft and guard Jeremiah Sirles on the physically unable to perform list. Kroft is in jeopardy of missing the start of the season after having surgery in May to repair a broken foot he hurt in practice.

The Associated Press contributed to this report.