The actress sparked concern when pictures emerged of her wearing a walking cast at Keith's drive-in concert on Friday, May 15.
During an interview with Entertainment Tonight, the country singer revealed the reason for Nicole's new accessory, telling the outlet on Saturday: "She broke her ankle, but she's a trooper."
"There she was last night among all of the folks, hobbling around on her boot with her mask on."
He gave no further details of how his wife had injured herself.
Nicole shared a snap from Keith's Nashville concert on Friday, but cropped it so as to not show her injured leg.
"Keith just had to play! Isolating with live music at last night's first #UrbanUnderground drive-in gig for the incredible #frontline workers," she wrote alongside the picture shared on Instagram, of her checking out the stage and screen at the Stardust Drive-In movie theatre.
The 52-year-old singer invited around 200 people from the nearby Vanderbilt University Health in Tennessee to join him at the venue, and later shared a video from the event on his Instagram page, writing: "A huge THANK YOU to all @vanderbilthealth frontline workers who attended tonight's special show at the drive in!".