DOVER, Del. Sprint Cup driver Brian Vickers is being administered drugs to dissolve blood clots, Red Bull Racing general manager Jay Frye said Friday at Dover International Speedway.
Vickers will be replaced in the No. 83 Toyota this weekend by Casey Mears.
Frye said Vickers has been given a clean bill of health, although doctors will not allow him to race.
Frye said Vickers, 26, complained of pain while preparing for appearances in Washington, D.C., on Tuesday and Wednesday. He was admitted to a hospital after consulting with a physician.
Multiple clots were found in blood vessels in a leg and around the lung, Frye said.
There is no timetable for his return, but Frye said Vickers could be released from the hospital as early as Friday.
"He's got great reason for optimism that everything's going to be fine," Frye said.
"It is a minor setback in a young man's life and career," Frye said. "Obviously, it's a major setback for our race team this weekend."
Jimmie Johnson said Vickers sounded like his usual self when they spoke Thursday night.
"There's still a lot of question marks and concerns and trying to understand just what in the world is going on," Johnson said. "Until they can find out what's going on, there's a lot of worry and concern."
NASCAR spokesman Ramsey Poston said Vickers must be cleared by the sanctioning bodys physicians before being allowed to race. Frye said his cursory knowledge of the condition or possible causes he Googled it makes him think the daily hazards of racing might have contributed.
Mears was dropped from the Richard Childress Racing lineup when the team downsized after last season. He has made two starts, with Tommy Baldwin Racing and Keyed-Up Motorsports.
Mears will apparently drive the No. 83 Toyota for as long as he's needed. Mears was also standing by to relieve Denny Hamlin at Phoenix, shortly after Hamlin had knee surgery, but wasnt needed.
"Our focus at this point is to get through the weekend, Frye said. Casey has agreed to do it as long as we want him to do it."
Missing the race will make another Chase qualification extremely unlikely for Vickers. He qualified for the first time last year with the help of a win at Michigan, but faltered in the final 10 races and finished 12th. Vickers has just three top-10s and is currently 20th in points.
The Associated Press contributed.