"It's not an indictment on David Carr by any stretch. I don't feel he's 100 percent," Fox said. "He's healthy enough to play, but I don't think he's 100 percent."
Carr practiced Friday and is listed as probable.
Testaverde said on Monday he thought Carr would start. Carr said the same on Wednesday, when Fox said he would play, but not necessarily start, if he was healthy.
But Fox said he didn't think Carr had progressed enough during the week, after sustaining a compression fracture in his lower back against New Orleans on Oct. 7.
"It's not a season-long decision. It will be game to game," Fox said.
Testaverde was out of the league after being cut by New England before the start of the regular season. The Panthers called him when they learned Jake Delhomme would be lost for the season to an elbow injury and after Carr hurt his back.
Testaverde flew into town Oct. 10 and learned enough of the offense to start four days later against Arizona. He then turned in a nearly flawless performance, completing 20 of 33 passes for 206 yards, including the go-ahead 65-yard touchdown pass to Steve Smith in the fourth quarter of a 25-10 win.
Testaverde, who turns 44 next month, did not throw an interception and became the oldest starting quarterback to win a game. He also extended his own record by throwing at least one touchdown pass in 21 consecutive seasons.
Fox then declined to say Carr would start if he was healthy, fueling a quarterback controversy last week when the Panthers had a bye, and earlier this week when Fox again held off on naming a starter.
"Contrary to popular belief, this wasn't all about keeping our opponent in the dark as much as it was us sorting it out," Fox said. "I think we have two starting quarterbacks, albeit not our original starter."
Carr has completed 52 percent of his passes with two touchdowns and two interceptions.
Running back DeShaun Foster (toe) practiced for a second straight day Friday and is listed as probable. Linebackers Dan Morgan (Achilles' tendon) and Adam Seward (calf) will not play.
AP Sports Writer Steve Herman in Indianapolis contributed to his report.