Marion told reporters after Wednesday morning's shootaround that he wouldn't have been able to play at that moment. He'll continue to get treatment on the ankle, which he sprained in the first quarter of Monday's win over the Milwaukee Bucks.
If Marion can't play, the Mavs would be without their top two wing players. Josh Howard (rehab from ankle surgery) remains out indefinitely.
Quinton Ross, a defensive-minded player with a limited offensive game, would likely start at small forward if Marion can't play.
That makes the Spurs an extremely tough matchup with Manu Ginobili and Richard Jefferson. Mavs rookie Rodrigue Beaubois has been excellent as a fill-in starting shooting guard, but coach Rick Carlisle has kept him in a limited role.
Tim MacMahon covers the Mavericks for ESPNDallas.com. E-mail firstname.lastname@example.org.