Testaverde, who showed up for practice on Saturday with a stiff back, is the third emergency quarterback for the Panthers.
Testaverde had appeared to have recovered from a strained right Achilles tendon that forced him to miss a game and leave another at halftime. Testaverde had started the previous two games and had practiced all week.
A team official said Testaverde was not injured at practice on Friday, but awoke Saturday with back pain.
Fox said Wednesday that Testaverde would start ahead of Carr, who has recovered from two concussions and a back injury.
Carr, signed to a two-year, $6 million deal in the offseason, has struggled to find receivers downfield and has been sacked 12 times. He's thrown three touchdown passes and three interceptions, and has left two games with injuries.
Carr had replaced Jake Delhomme, who was lost to a season-ending right elbow injury in Week 3. Undrafted rookie Matt Moore has also played briefly in four games.
The Associated Press contributed to this report.