McNabb, a six-time All-Pro selection who enjoyed a 13-year NFL career, was one of three panelists on the Fox Sports 1 program, "Fox Sports Live."
Asked where he felt Johnson ranked among top current athletes, McNabb said third, but added he didn't believe the Hendrick Motorsports driver was an athlete.
"You've got to like what he's been able to do. You've got to give him a little credit," McNabb said. "Do I think he's an athlete? Absolutely not."
A somewhat startled host, Charissa Thompson, quickly asked McNabb for clarification.
"You don't think he's an athlete?" she asked.
"Absolutely not," McNabb responded. "He sits in a car and he drives. I give credit to what he's been able to do ... you look at the list and for me it's Tiger Woods, Kobe Bryant, Derek Jeter, you talk about Tom Brady. I think the things (Johnson's) been able to do I give him a lot of credit for. I ranked him third behind Tiger Woods and Kobe Bryant. You give credit to Tiger for what he's been able to accomplish; Jimmie Johnson -- congratulations."
While fans as well as others involved in the sport called out McNabb for the comments, Johnson didn't take to social media until Saturday morning to respond.
Via his Twitter account, Johnson posted the following items:
The debate continues... Everyone is entitled to an opinion. #DriversAreAthletes— Jimmie Johnson (@JimmieJohnson) November 16, 2013
Johnson, who will be chasing a sixth career NASCAR Sprint Cup title Sunday at Homestead-Miami Speedway, is considered one of the most physically fit drivers in NASCAR, often working out twice a day with routines that include long runs, mountain biking and swimming.
Earlier Friday, team owner Rick Hendrick, whose Hendrick Motorsports organization fields the No. 48 entry, said Johnson's physical fitness has been a huge plus as his driver continues to contend for championships and pile up race victories.
"I think it's disrespectful to say he's not an athlete, fellow panelist and former U.S. Open tennis champion Andy Roddick said.
"Is he an athlete?" McNabb asked.
"I think he is," Roddick replied. "I think he's one of the best athletes from the last decade. If you look in the context of individual sports, you've seen (Usain) Bolt become the best sprinter of all time; we've seen Michael Phelps become the best swimmer of all time. ... Tiger Woods, he's chasing the major record; Roger Federer has broken a major record and Rafael Nadal is right on his heels.
"Jimmie Johnson is doing the same thing. He's breaking all-time records or he's on his way to breaking all-time records ... I have a lot of respect for Jimmie Johnson.
"Asked if he was saying NASCAR wasn't a sport, McNabb replied: "I didn’t say it wasn't a sport. I said he's not athletic. So when you put him in an athlete group ..."
Johnson enters Sunday's race with a 28-point advantage in the Chase for the Sprint Cup points battle and needs only to finish 23rd or better to lock up his sixth championship.
Only two other drivers, Richard Petty and Dale Earnhardt, have won more titles with each capturing seven during their respective careers.
UPDATE: McNabb responded to some Twitter backlash late Saturday afternoon.
Glad to see that you are watching @FOXSportsLive. If you have any questions to what I said, look up the word and you will have your answer.— Donovan McNabb (@donovanjmcnabb) November 16, 2013