Marcus Smith had been a quarterback at Hardaway High School in Columbus, Ga., and the University of Louisville had recruited him to be a quarterback when Smith decided to play there. When he arrived on campus in the fall of 2010, the coaches moved him to linebacker. In 2011, they moved him to defensive end.

He was still a defensive end last year, when he finished second in the country with 141/2 sacks. His role and purpose on the field became instinctual to him.

"Every time, I had the whole game to rush the passer," he said Tuesday. "So just that one time that you beat your guy, it could change the whole game."

The Eagles selected him in the first round of this year's NFL draft. Because defensive coordinator Bill Davis prefers a 3-4 alignment that requires stouter defensive ends, and because Smith is a relatively lean 6-foot-3 and 251 pounds, the Eagles moved him to outside linebacker. When Mychal Kendricks, one of the team's starting inside linebackers, injured his calf two weeks ago, Smith replaced him.

That winding backstory, with Smith's ricocheting from one position to another, accounts for how he came to change a whole game Sunday - for all the wrong reasons.

On the first play of the second quarter, Smith dropped into zone coverage when 49ers quarterback Colin Kaepernick went back to pass. But once Kaepernick scrambled to the left, Smith was supposed to "plaster" the 49ers receiver nearest to him, running back Frank Gore. That is, Smith should have covered Gore man-to-man. He didn't. He followed his instincts and followed Kaepernick, who stopped, swiveled his hips, and chucked the football across the field to Gore for a 55-yard touchdown, the 49ers' first in their 26-21 victory.

"As soon as the ball was thrown, I was like, 'Oh, snap. That's my guy,' " Smith said after the Eagles practiced Tuesday. "Gore stepped up and blocked, so I kind of went with all the chaos. But I've got to be able to settle down and come back down to earth and know that Gore is my guy.

"After the play, people were telling me to keep my head up and keep playing, because that was a big factor. If that play wouldn't have happened, we probably would have won."

Smith's mistake was just one of several regrets for the Eagles out of Sunday's loss: Riley Cooper's end-zone drop, Cary Williams' untimely defensive-holding penalty, those two Nick Foles incomplete passes from the 11/2-yard line in the closing minutes. But Smith probably wouldn't have been on the field at all had Kendricks been healthy, and his uncertain play has reinforced the perception that the Eagles outsmarted themselves by taking Smith, whom most draft analysts projected to be a second- or third-round pick, with the 26th overall selection.

Some of that criticism is fair, but only some. It is reasonable to argue that, given their weaknesses on defense, the Eagles should have drafted a player in the first round who was capable of stepping into the starting lineup immediately. It is unreasonable to argue that Smith, after four NFL games and even after his mental error Sunday, will never develop into such a player.

"It's just about me now being comfortable and making plays without second-guessing myself," he said. "I think that's what makes me play slow sometimes. I just want to be correct for the team. I don't want to mess up. Coach Davis says it all the time. 'Don't worry about that.'

"That's the biggest thing for me as a first-rounder. I know people want me to play now. They expect me to be better than I am right now. But I'm trying to work hard, trying to get there, and once I get to that point, I'll never look back."

Smith admitted that quelling his impulse to pass-rush, that training his eyes to follow a running back or tight end instead, has been his greatest adjustment. He is not the only end-turned-linebacker in Davis' system who has felt that way.

"I know it was a challenge for me," said Trent Cole, who moved to linebacker last year after playing defensive end for his first eight NFL seasons. "It's hard. It still is hard. If the quarterback rolls out, you've got to stay in man coverage or zone coverage. You want to go after him, but you can't. You've got to be patient and disciplined.

"Patience and discipline is the key, especially for a guy who was built to freaking go after the quarterback."

Smith will need both qualities as long as he stands in for Kendricks, who has missed the Eagles' last two games and didn't practice Tuesday, and he will need them thereafter. He might remain at inside linebacker. He might end up back on the outside.

"Sometimes," he said, "I wonder or ask myself, 'Can I just get really good at one position? Can I master one?' " Neither he nor the Eagles know yet, but Marcus Smith is certain of this: He'd like the chance to change a whole game with one play again, this time the way he used to.