The Houston Texans fear starting running back Lamar Miller suffered a torn left ACL during Saturday night's preseason game against the Dallas Cowboys, a source told ESPN's Adam Schefter.

He will undergo an MRI to confirm the tear.

Miller was taken off the field on a cart during the first quarter in Arlington, Texas.

On first down, Miller was tackled by Cowboys tackle Maliek Collins, who hit him just above the left knee. Miller, 28, grabbed his left leg and was checked out by the medical staff. He was eventually carted off the field.

"It was a tough thing to see," Texans coach Bill O'Brien said of Miller's injury. "It's very difficult to lose a guy like Lamar, for a lot of reasons. He's a great pro. He's just an awesome guy in the locker room."

Quarterback Deshaun Watson echoed his coach.

If Not Miller, Then Who? After Lamar Miller, the Texans have just three running backs listed on their depth chart who have recorded a rushing attempt in the NFL; they've combined for 1,494 career yards.

In NFL history, there have been 87 instances of a running back rushing for more than 1,494 yards in a single season. It has been done by 54 different backs. Texans RB Career Rush Yards Duke Johnson Jr. 1,286 Taiwan Jones 183 Josh Ferguson 25 Damarea Crockett 0 Cullen Gillaspia 0 Buddy Howell 0 Karan Higdon Jr. 0 TOTAL 1,494 -- ESPN Stats & Information research

"It's very tough. You have a guy that is so special to our team," Watson said. "[When] anybody gets hurt, it's tough. But he's one of the guys that has a lot of energy and a lot of power for this team. And, I mean, it was tough to see him go off on the cart."

Miller has been the Texans' lead back since 2016. In 14 games last season, he ran for 973 yards on 210 carries and scored five touchdowns. Miller is entering the final season of a four-year contract.

Houston traded for running back Duke Johnson earlier in the month but was hoping to feature both backs in its offense. Miller was replaced in Saturday's game by Taiwan Jones.

O'Brien said he isn't sure yet how losing Miller will affect playing time for Johnson but said, "I think our backfield situation with depth is actually decent."