Mason will not play in St. Louis' final preseason game and is "questionable" for the season opener against the Seattle Seahawks.
"It was just a little tweak," Mason said after Saturday's game, via the St. Louis Post-Dispatch. "It's nothing too bad. It's just tight. It got a little sore, so got out of there."
Mason missed time earlier in training camp with a similar hamstring injury, which can be unpredictable and tend to linger.