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The San Antonio Spurs received some encouraging news Monday afternoon when an MRI exam came back negative on Tim Duncan's right knee and right ankle, both injured in the first half of Sunday's home victory over Portland.

Duncan has been ruled out of Wednesday's showdown with San Antonio's longstanding rivals from Dallas, but Spurs spokesman Tom James said that a firm diagnosis on how long Duncan will be out is at least a day or two away.

The Spurs say they want to see how Duncan responds to treatment before putting an estimate on his return date. But they are treating it as a short-term injury, which naturally comes as a great relief to the defending champs. Things didn't look especially promising when Duncan went down hard after tangling with Portland's James Jones in the second quarter Sunday, with the two-time MVP requiring the help of teammates to leave the floor.

"It's not a big deal at all," James said.

"It's more scary than anything else -- at least that's what I'm hoping," Spurs coach Gregg Popovich said Sunday.

Although Duncan's right knee is still sore, he'll be listed as inactive with a sprained right ankle Wednesday when the Mavericks visit the AT&T Center for the first time season. The Mavs won the teams' first meeting in Dallas on Nov. 15.

Even if Duncan winds up missing a week or two, San Antonio has a favorable schedule in December to cope with his absence. The Spurs are one of only two teams in the league still unbeaten at home this season at 10-0 -- Boston is the other at 9-0 -- and will be on the road for only three of their 12 games this month.

Marc Stein is the senior NBA writer for ESPN.com. To e-mail him, click here. Information from The Associated Press is included in this report.