Peterson was a healthy scratch for the Redskins' season opener against the Philadelphia Eagles. Second-year runner Derrius Guice got the start in his NFL debut after missing his entire rookie campaign with a torn ACL.
However, Guice was ineffective and then sustained another knee injury. The LSU product is now expected to miss an extended period with a meniscus issue, according to ESPN's Jeremy Fowler.
Gruden had previously seemed less than enthusiastic about the idea of going with Peterson at running back.
"I think it will be week-to-week," Gruden said about Peterson's playing time heading into the season. "So if we have a game where we think we can run the ball 55 times a game in an I-formation, then sure, I'll get him up."
With Guice now sidelined, Peterson and Chris Thompson - who is more of a third-down specialist - are the team's options at running back.
When Guice's ACL tear thrust Peterson into the starting role last year, the 34-year-old responded with a revival season, posting 1,042 yards and seven touchdowns on 251 carries after amassing just 602 yards in the previous three years combined.