The first call A.J. Jenkins made after he was selected by the 49ers in the first round of last month’s NFL draft was to Paul Petrino, his offensive coordinator at the University of Illinois, his “second father” and his toughest critic.

Through a jumble of tears and laughter, Jenkins thanked Petrino for transforming him from an afterthought into an all-Big Ten wide receiver.

On the other end, Petrino, 44, in a St. Louis hotel room on a recruiting trip, laughed along with his former pupil, sharing a moment made possible by their two years of toil together.

Near the end of the conversation, though, Jenkins learned that Petrino’s pats on the back would still be mixed with plenty of kicks in the pants. Jenkins’ life had just changed, but one thing remained unaltered: Coach P would still demand his very best.

“I told A.J., ‘Go do it now,’ ” Petrino recalled of his parting words. ” ‘Make sure you get a run in tomorrow. Don’t relax. And make sure you have your ass in shape for minicamp.’ ”

As with most first-round picks, Jenkins will enter the NFL dealing with high expectations, a burden he’s uniquely equipped to handle, thanks to his relationship with Petrino. Jenkins had 90 catches for 1,276 yards and eight touchdowns as a senior, ranking among the top four in school history in each category. But a 13-game performance that earned him a lofty spot in the record books routinely fell short of Petrino’s standards.

School record, but …

Consider, for example, last October when Jenkins strung together the best back-to-back games by a wideout in school history. In a win over Northwestern, he had 12 catches for a school-record 268 yards and three touchdowns. He followed with six catches, 182 yards and touchdowns of 77 and 67 yards in a win over Indiana.

He was the nation’s leader in receiving yards and the toast of Champaign. And he was also in Petrino’s doghouse due to a second-half drop in that 21-point win over Indiana that the coach attributed to a lack of focus.

“The biggest thing (A.J.) needs to do is show up every day at practice, work as hard as he can, not get too worried about reading the newspaper and reading about himself too much,” Petrino told reporters after the game.

Whoa. How did Jenkins handle such a critique? In a phone interview from his hometown of Jacksonville, Fla., Jenkins laughed at the memory.

After that record-setting performance against Northwestern a week earlier, he said, Petrino delivered another message as he exited the interview room where Jenkins was holding court with the media.

“You’ve got to practice hard every day,” Petrino shouted. “Then you’ll keep it up.”

Jenkins concedes Petrino’s never-satisfied style inspired a few moments of frustration. But the three-time Big Ten All-Academic selection understood why Petrino would review one of his touchdown catches and ignore the end result, picking apart the sloppy footwork or lazy shoulder drive that preceded it.

“He gets the best out of players like that,” Jenkins said. “He’d go after me even harder in practice after I had a really good game. That’s him. Just get over it. You can live in the glory of a play for so long, and then it’s time to move forward. And that’s what I’ve learned. He’s always telling me to stay hungry. Stay humble. He told me that when I first met him – he’s never satisfied.”

Almost left for Florida

When the duo first met at Illinois on Dec. 14, 2009, they were heading in opposite directions.

Petrino had been introduced as the new offensive coordinator of a team nine days removed from finishing a 3-9 season. His first task in his new job: keep Jenkins from leaving.

Petrino left his introductory news conference and headed upstairs, where Jenkins and his dad, Al, were sitting outside Ron Zook’s office. They were waiting for the Illini head coach to sign a release so Jenkins could transfer to a school closer to home, perhaps Florida. Jenkins was both homesick and sick of his puny role in an offense in which he had 10 catches for 123 yards as a sophomore.

Before signing a release, Zook wanted Jenkins to meet the well-regarded Petrino, who had commanded a salary of $475,000, making him the highest-paid assistant in the Big Ten.

In his initial meeting with Jenkins, Petrino began earning his money, first detailing his work with wide receivers. As Louisville’s offensive coordinator from 2003-06, he coached Harry Douglas, who became a third-round pick in the NFL draft. As the wide receivers coach with the Atlanta Falcons in 2007, he coached Roddy White, who had the first of his five consecutive 1,000-yard seasons.

Petrino also showed Jenkins film of his offenses at Louisville and Arkansas, where he coached from 2008-09. Louisville averaged 41.1 points in his four years at the school. Arkansas ranked ninth in the NCAA in scoring in 2009 (Illinois ranked 81st).

Al Jenkins, who had wanted his son to return to Florida, was impressed by Petrino’s resume and non-groveling pitch.

“He was kind of tough and rugged, and I felt like A.J. needed that anyway,” Al Jenkins said. “I said, ‘Hey, buddy, if this is where you want to be, let’s give this a second look.’ ”

Jenkins had just 21 catches in his first two seasons, but Petrino, who had recruited him out of high school, quickly recognized he possessed NFL ability. He had soft hands and effortlessly caught balls away from his body at awkward angles. He also had a coveted combination of speed and quickness, allowing him to move in and out of his routes smoothly without decelerating.

Restoring swagger

What he didn’t have, however, was confidence. It was a must-have trait for a receiver in Petrino’s mind, but Jenkins had lost his swagger after playing a bit role in the Illini’s offense. Before kicking him in the pants, Petrino began their relationship by patting him on the back.

“Coach Petrino sat A.J. down after winter workouts and told him he could be one of the best receivers, if not the best receiver, ever to come out of Illinois,” Illini quarterback Nathan Scheelhaase said. “And A.J. took that to heart for sure. He believed in it.”

Jenkins bonded with Petrino as he realized his ultra-demanding, high-decibel coach had a genuine affection for his players, from the stars to third-stringers Jenkins said Petrino treated like family. Jenkins developed a close relationship not only with Petrino, but with the coach’s 14-year-old son, Mason.

In December, Petrino left Illinois to become the offensive coordinator at Arkansas, but his texts and phone calls to his former pupil didn’t stop as Jenkins readied for the NFL draft. And now that he’s a first-round NFL pick, Jenkins doesn’t expect the messages – stay hungry, stay humble – to stop pouring in.

He may never reach his coach’s standards, but he knows from experience what’s possible even if he falls short.

Twenty-eight months ago, after all, he stayed at Illinois. But he didn’t stay the same.

“I know no matter what I do on that field, I can always do something better,” Jenkins said. “And I guarantee he’ll always let me know that. When I call him, he’s going to let me know. I love him for that.”

COMING MONDAY: A look at the Raiders‘ draft, including the Stefen Wisniewski-Penn State connection.