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Pittsford, N.Y. • Buffalo Bills wide receiver Stevie Johnson left practice after injuring his left hamstring Friday morning.

Johnson, Buffalo's top receiver, pulled up on a route during a red zone drill two hours into practice. He was evaluated by Buffalo's training staff and walked slowly to the locker room assisted by two people.

Bills coach Doug Marrone said after practice that he did not know the severity of the injury and that Johnson will be evaluated by Buffalo's doctors.

"It would be a major concern when we talk about it during the season, but right now, I think a veteran player that has proven himself, we'll just see what the doctors say and it is what it is," Marrone said. "We'll deal with it from there."

This is the second training camp in a row that Johnson has dealt with an injury. Last year, Johnson was battling his way back from offseason groin surgery. Johnson led the Bills last year with 1,046 yards receiving and is coming off three consecutive 1,000-yard seasons.

Tight end Mike Caussin also left practice Friday with a hip injury.