PITTSFORD, N.Y. — Buffalo Bills running back LeSean McCoy hurt his left hamstring Tuesday night and is having tests to determine the extent of the injury.

Coach Rex Ryan said trainers informed him the hamstring is intact and has not been pulled off the bone, which he described as “a real positive.” The injury occurred about an hour into a joint practice with the Cleveland Browns.

Ryan didn’t want to be pinned down on a timetable for how much time McCoy could miss.

“I really don’t want to go there yet, because let’s get the prelim reports,” he said. “Hopefully, this will be something that doesn’t set him back too far.”

McCoy was running a route during a 7-on-7 red-zone drill when he suddenly pulled up short near the goal line. He slowly walked to the sideline holding the back of his left leg and then sat on the turf, where he was attended to by numerous trainers. He required help getting up, and didn’t bend his left leg while slowly making his way to the locker room alongside team doctor Les Bisson and trainer Shone Gibson.

The injury could be a big blow for the Bills after they acquired the two-time All-Pro in a trade that sent linebacker Kiko Alonso to Philadelphia in March.

McCoy led the NFL with 1,607 yards rushing in 2013, and was regarded as the centerpiece in Buffalo’s plan to establish a run-first offense under Ryan. The former New York Jets coach took over the Bills in January, replacing Doug Marrone, who stepped down on Dec. 31.

Buffalo is likely down to just two healthy running backs — undrafted rookies Bronson Hill and Ricky Seale — for its preseason game at Cleveland on Thursday night. Seale had just signed earlier in the day after Karlos Williams was taken to a hospital and had an undisclosed medical procedure on Monday.

Without discussing the nature of the procedure, Ryan said Williams will miss about two weeks.

“I hope they’re quick learners,” Ryan said, referring to Hill and Seale.