SANTA CLARA, Calif. (AP) — Ahkello Witherspoon had a 4.4 GPA in high school and aspires to go to medical school once his football career is done.

But he wasn't a quick study when it came to adjusting to the 49ers.

The rookie cornerback was inactive the first four weeks of the season. After Tuesday's trade of Rashard Robinson to the Jets for a draft pick, Witherspoon is San Francisco's newest starter on defense.

The 49ers felt comfortable with Witherspoon's evolution on the practice field from starry-eyed third-round pick to a cornerback playing as if he belonged.

It was evident during the week even while he was a healthy scratch during the first quarter of the schedule.

"I used those four weeks as another minicamp," Witherspoon said. "I played better those four weeks than I did throughout training camp coming into the season."

Witherspoon cemented his starting job last week against the Eagles, two days before Robinson was shipped to New York for a fifth-round pick.

Witherspoon in the second half logged his first career interception and returned it into Eagles' territory, setting up the 49ers' only touchdown during the 33-10 loss.

"For a rookie to go in and study the way he does and really put himself in that position," defensive coordinator Robert Saleh said. "He ran the route for (the receiver). He knew it was coming."

Witherspoon did well in coverage throughout the day, only allowing three catches on eight targets, according to Pro Football Focus.

However, he was in coverage against receiver Alshon Jeffery when he hauled in a 53-yard touchdown pass from Carson Wentz.

It was a learning experience for the Colorado alum who was burned for the score on the possession that followed his forced turnover.

"I could have intercepted it," Witherspoon said. "On that one, I kind of second-guessed myself. And once you do that, you lose at the corner position, as we saw. And to have success, you have to believe in yourself at all times. One thing I learned from that game is just stay on the same mindset and just continue to dominate mentally."

Witherspoon is 6-foot-3 and fits well in Saleh's scheme that relies heavily on physical play from its cornerbacks. Witherspoon ascended to the starting lineup, in part, because he was a draft pick of the new front office headed by general manager John Lynch.

Robinson was a holdover from the previous regime — and he also led the team with 10 penalties before getting traded.

Saleh said earlier in the season that Witherspoon was getting physically stronger. But it was the strides he was making mentally that allowed him to become a starter halfway through his rookie campaign.

"He hasn't even scratched the surface, either. He's got a great amount of potential," Saleh said. "It's just whether or not he keeps that same mindset of growth."

NOTES: WR Pierre Garcon missed practice Thursday for a second straight day with a neck injury . LT Joe Staley (eye), OG Brandon Fusco (biceps), DL Solomon Thomas (knee), DB K'Waun Williams (quad), DE Aaron Lynch (calf), DL D.J. Jones (knee), OT Garry Gilliam (knee) also sat . RB Matt Breida (knee) and LB Reuben Foster (ribs) were in non-contact jerseys.

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