Jackson Alexander of MLB.com has the info:
It occurred when Ellsbury took an awkward step around first base, legging out a third-inning double.
Despite the injury, manager Joe Girardi didn't sound too worried about it. Ellsbury was able to remain in the game, which looking back on it was risky at best.
Brendan Kuty of NJ.com gives us a quote from Girardi that leads us to believe Ellsbury could have played, but was held out as a precaution:
"But he's played a lot of days in a row here," Girardi said. "So I figured with the consistent rain there would be tonight and the soft outfield I'd give him a day. But I'd expect him to be there (Tuesday)."
The best part about that quote was the fact that Girardi expects Ellsbury to be ready on Tuesday. In the same report from Kuty, Ellsbury also confirmed he should be in the lineup versus the Seattle Mariners.
Ellsbury is seeing the ball as good as he has in 2014 with an ongoing 13-game hitting streak. The Yanks could certainly use him in the lineup as their offense continues to struggle mightily.