Tre Mason's right hamstring pull is "mild."
Tre Mason was forced from Saturday's preseason game with a right hamstring injury.
According to the St. Louis Post-Dispatch, second-year RB Tre Mason is expected to get the "lion's share" of the Rams' carries "early in the season."
The Rams' official website expects Tre Mason to "still play a significant role" in 2015, and believes it's "more probable" Mason starts Week 1 than Todd Gurley.
Tre Mason has slimmed down to between 195 and 200 pounds.
ESPN Rams reporter Nick Wagoner confirms Tre Mason "looks to be entrenched" as the starting running back.
Rams coach Jeff Fisher says Tre Mason sat on the bench for the first four games of 2014 because he needed to "learn how to play without the football."
Tre Mason rushed for 28 yards on 11 carries in the Rams' Week 17 loss to the Seahawks, adding three catches for 29 yards.
Tre Mason rushed 13 times for 76 yards with a touchdown in Sunday's Week 16 loss to the Giants.
Tre Mason rushed 13 times for 33 yards in the Rams' Week 15 loss to the Cardinals, also losing a fumble.
Tre Mason rushed 20 times for 66 yards in the Rams' Week 14 win over the Redskins.
Tre Mason rushed 14 times for 117 yards and two touchdowns, and caught 3-of-4 targets for 47 yards and an additional TD in the Rams' Week 13 blowout win over the Raiders.
Tre Mason rushed for 62 yards on 16 carries in the Rams' Week 12 loss to the Chargers.
Tre Mason rushed for 113 yards on 29 carries in the Rams' Week 11 win over the Broncos.