The pair combined for 43 points last week in leading the Mystics to a nine-point opening-game win, but both were listed as questionable with back spasms as the series resumes on Sunday tied at a game apiece.
The 6-foot-5 Delle Donne, who averaged 19.5 points and 8.2 rebounds this season, is suffering from a herniated disk that is pinching a nerve in her back. She left Tuesday's 99-87 Game 2 loss without scoring a point after playing only a few minutes in the first quarter.
Thibault said that Atkins, who had 21 points on 6 of 7 shooting in Game 1, bent down at some point on Thursday night and could not stand up again.
Both players traveled with the Mystics to Connecticut, but were receiving medical treatment on their backs while their teammates practiced on Saturday.
''I'm not counting on them playing,'' Thibault said. ''I know (Connecticut Sun coach Curt Miller) thinks I'm making this stuff up. I wish I was. Our team is preparing to play tomorrow without either player.''
Without Delle Donne's inside presence, Connecticut dominated in the paint in Game 2, outrebounding Washington 41-27. Sun forward Jonquel Jones scored 32 points and grabbed 18 rebounds, including a Finals record nine on the offense end of the court.
Mystics forward Emma Meeseman, who scored 23 points Tuesday, said the team has made some defensive adjustments to deal with Jones if Delle Donne can't play on Sunday.
''It's going to be a team defense,'' she said. ''She has to see that it's one against five, not one against one.''
Delle Donne has dealt with illnesses and injuries throughout her career. She has battled chronic Lyme disease since college and had back issues that also limited her in the Finals when she played in Chicago in 2014.
Miller said he believes that in a best-of-five Finals she will do whatever is necessary to play on Sunday.
''We'll see how 100% she is, but she's going to play,'' Miller said. ''This is a big moment for their franchise. It's a big moment for her. She's going to play and we're prepared for that.''
This will be the first WNBA Finals game at the Mohegan Sun arena since Thibault coached the Sun and led them to back-to-back Finals in 2004 and 2005. Sunday's game is sold out.
Both teams and both coaches are looking for their first titles.
Sun guard Courtney Williams, who has averaged 24 points over the first two games of the Finals, was asked if it would mean any less to win it with Delle Donne on the sidelines.
''If we win, we win; I don't care how we win'' she said. ''If she plays, if she don't play. It doesn't matter to me, as long as we win.''