Foster had a league-leading 55 carries in the first two weeks of the NFL season, but he was prevented from adding to his total on Sunday due to the strain.
"I've played through a lot in this league, and hamstrings are the hardest thing to gauge because you don't know how it's going to fatigue, how it's going to react to a certain cut," Foster said, ESPN reports.
"You've just got to let it rest... having things heal on their own time. You can't push it or it'll relapse on you."
The 28-year-old revealed that he found it particularly difficult looking on from the sidelines after an injury-plagued season last year, but he was "proud" of rookie Alfred Blue's performance in his absence.