"I was shocked, that was my sense of it. I was just shocked," Young said Wednesday, recalling the blow from blitzing cornerback Aeneas Williams that knocked him out midway through San Francisco's 24-10 victory at Arizona on Monday night.

	Niners quarterback Steve Young is motionless on field as teammate Derrick Deese calls for assistance.

Young will miss Sunday's game against Tennessee on doctor's orders, and backup Jeff Garcia will make his first NFL start in his place.

"I remember a flash and as I was going backwards, catching (tackle) Dave Fiore's knee and then the ground," Young said. "And then mostly just resting for a second because I wanted to collect myself. But once I stood up, I felt I knew exactly what was going on and that's why my initial reaction was to go back into the game."

Young didn't get back in and, after undergoing tests Tuesday, Stanford neurologist Dr. Gary Steinberg recommended Young sit out at least a week, and possibly two.

"Basically, what he said was that you don't want these things bunched up," Young said. "As long as you give your brain a rest and there are not a tremendous amount of symptoms or things that go on that would cause more concern."

Tests showed no signs of additional brain trauma or cumulative damage from the three concussions Young had between October 1996 and August 1997.

"The MRI did not show any of the so-called white spots, which are thought to be indicative of progressive brain trauma," Young's agent, Leigh Steinberg, who is not related to the neurologist, told ESPN. "So, that was clear. But the doctor did recommend he sit out at least a week and possibly two."

The dangers from repeat concussions over a period of time include premature senility and an increased risk for Alzheimer's disease and Parkinson's disease.

"He will participate in practice but will not play against Tennessee," 49ers coach Steve Mariucci said of Young. "We will re-evaluate Steve's status next week."

Garcia, who spent five years in the CFL, will now be making his first NFL start on Sunday.

"It's definitely something I kind of anticipated," Garcia said. "It's something I'm prepared to face. I'm not trying to replace Steve Young. I'm just trying to help this team get a win."

Wide receiver J.J. Stokes said he wants Young to come back but his health comes first.

"I think everybody is concerned about him," Stokes said. "His family is probably most concerned but at the same time, we're concerned about Steve, too. He has to worry about himself first, then worry about us second."

Young, who considered retiring after the last series of concussions and was urged to do so by his family, said he was encouraged by the doctor's findings and plans to resume playing as soon as possible.

He downplayed any thoughts about retiring after the latest episode.

"I feel pretty good," he said. "Yeah, it was a ferocious hit. The week before, there were some pretty tough hits. They do come with the game and I do enjoy the game."

But he also said he took the issue of concussions seriously and in all likelihood would follow doctors' advice should they at some point recommend against a return to football.

"We would just act accordingly," Young said. "I've said this from the beginning. If that was the discussion, we would take it to its end. I'm unafraid to take in all the possibilities, if that's where the doctor says it is.

"But everything was negative (in the tests), the doctor said to take a week off, so there we have it. I'm just going to have to miss this game. That's the main thing. I'll talk to the doctor next week and we'll go from there."

The so-called second-impact syndrome, in which a person suffers a second concussion while still involved with the first, could be life-threatening, regardless of the severity of the second concussion.

Leigh Steinberg told ESPN that Young is not in denial over the concussions he's suffered, and had Dr. Steinberg told him on Tuesday that he would suffer permanent damage if he continued to play, Young would have walked away from the game.

Steinberg also said that two years ago, he recommended that Young retire. But now, he said doesn't feel the need to address that question again.

"He will follow the medical advice," Steinberg said. "He obviously has tremendous opportunities for life after football and he's not going to endanger that."