Romo was limping as he carried out fakes on running plays against Washington. He also appeared to kick the back of his leg after spinning free from the pocket on a third-down pass to Miles Austin.
Romo completed 17 of 27 passes for 226 yards, including the winning touchdown to DeMarco Murray on fourth-and-goal from the Washington 10 with 1:08 to play.
Longtime teammate Jason Witten admired Romo's toughness after news of the extent of the quarterback's injury had been learned.
"A lot of times you lose sight of [Romo's toughness], what Tony is all about, the kind of toughness that he shows day in and day out, not only mentally but physically, you just commend him how he approaches it, just resilient to find a way and he's so good in those situations and kind of put the team on his back."
Romo has dealt with numerous injuries over the years, including a broken rib in 2011. He hurt his ribs again in last season's season-ending loss to the Redskins, a result that kept the Cowboys from making the playoffs.
ESPN.com Cowboys reporter Todd Archer and ESPNDallas.com's Calvin Watkins contributed to this report.