The league's MVP left Game 2 of the WNBA Finals on Tuesday night after playing only a few minutes in the first quarter because of back spasms. She didn't return, and an MRI revealed the herniated disk Wednesday. Delle Donne is undergoing treatment and the team said it would update her status.
The series is tied 1-1, with Game 3 at Connecticut on Sunday.
Washington coach Mike Thibault said after the game that when Delle Donne went up for her first layup, her back ''grabbed on her,'' and she immediately asked to be subbed out.
The 6-foot-5 forward has dealt with illnesses and injuries throughout her career. She's battled Lyme disease during college at Delaware and it occasionally flared during her pro career. While she was playing with Chicago, Delle Donne dealt with back issues that limited her in the Finals in 2014.
Last season, she helped the Mystics reach the championship round despite suffering a bone bruise in her knee in Game 2 of the semifinal series against the Atlanta Dream. She missed the third game, but returned for the final two contests to help Washington make the WNBA Finals for the first time.
Seattle swept that series last year.
This season, she missed a few games during the regular season because of knee issues and a broken nose.