SANTA CLARA, Calif. -- Hard as it is to believe now, San Francisco 49ers tight end George Kittle wasn't exactly a hot commodity on the college recruiting circuit. As a senior at Norman (Oklahoma) High School, Kittle didn't generate much interest from most Power 5 schools.

Kittle did just enough to land on Iowa coach Kirk Ferentz's radar. As Ferentz was trying to gauge what kind of player Kittle could become, he turned to then-Oklahoma coach Bob Stoops. Bruce Kittle, George's father, was on the Sooners' staff and Stoops had gotten to know George.

During that conversation, Stoops and Ferentz ran through a variety of possibilities, including moving Kittle to linebacker or a hybrid role on the defense.

"But you know, he just he kept growing," Ferentz said.

The college growth spurt that followed wasn't limited to his height and weight. Kittle's physical growth was accompanied by a steady ascension as a player.

As it turned out, it was the start of a long process that has made Kittle one of the NFL's best and most versatile tight ends and an indispensable member of the 49ers' offense. Kittle, who missed the previous two games because of knee and ankle injuries, returned with a season-high 129 yards receiving in the Niners' 37-8 victory over the Green Bay Packers on Sunday night.

"He's on a superstar level," receiver Kendrick Bourne said. "He's a different level. And I'm definitely inspired by him."

When Kittle arrived on Iowa's campus, he was 6-foot-2, 201 pounds. In need of added size and muscle, he redshirted his freshman season and began a process that would see him grow another inch and add 50 pounds.

Since he wasn't counted on to contribute on game day as a freshman, Kittle worked on the scout team in a variety of roles. It was there Kittle embraced the idea that the more he could do, the better his chances of getting meaningful playing time.

It's also where Ferentz first saw signs Kittle would grow into the player he is now. Ferentz knew Kittle was a good athlete but noted his relentless motor. When Kittle would jump in with the scout-team offense, he gave the starting defense fits with his ability to get open and make contested catches.

Kittle's willingness to do the dirty work made for a natural progression, as Ferentz and his staff hammered home the importance of being not just a willing blocker, but a dominant one.

"We asked him to block as well as being good in the passing game," Ferentz said. "And it’s probably fair to suggest that the passing game came easier to him at the front end of his career. But, you know, just like he's got better in all areas, and certainly is a lot more physical, a lot more mature now than he ... it's not even close to what he was as a freshman."

Iowa strength and conditioning coach Chris Doyle still marvels at how Kittle managed to add weight so methodically as he gained about a pound a month every year for four straight years.

"It was steady," Doyle said. "It took a lot of diligence, and it just took time. Even when he went to the NFL, he was still developing. In my mind his best football was still ahead of him."

Last year, Kittle's star rose dramatically as he set a league record for receiving yards by a tight end (1,377) and posted the most yards after the catch in a season (870) since it began being tracked by ESPN in 2010. Because of that production, Kittle's blocking ability often gets overlooked.

Make no mistake, for as dangerous as Kittle is as a pass-catcher, he's an equally devastating run blocker. In fact, it's the part of the game Kittle values most.

"I will take burying a guy over a 40-yard explosive [catch] any day," Kittle said. "My favorite thing is moving a man from point A to point B against his will. It's the greatest feeling you'll have. There's not much better than taking a 280-pound defensive end and putting him on his back."

Through the first half of this season, Kittle's receiving numbers were still solid, with 46 catches for 541 yards and two touchdowns. But Kittle's ability to open running lanes on the edges cannot be overlooked as the Niners' run game surged.

San Francisco ranked first in attempts and third in yards and explosive runs (gains of 10-plus yards) through the first eight games with Kittle on the field. More often than not, it was Kittle wiping out a linebacker, cornerback or even a defensive end to pave the way to a big run, such as running back Matt Breida's 83-yard touchdown run against Cleveland in Week 5.

"That's where it starts for George," Bourne said. "It just shows he cares about each and every single play. It ain't that he's going hard because he's getting the ball. He's going hard because he cares about every play. He knows every play adds up to the end result."

If Kittle's value to the Niners wasn't already obvious, his two-week absence from the lineup only crystallized it further. Without Kittle, the 49ers suffered their first loss and narrowly edged the Cardinals in a rematch. The Niners' run game suffered, averaging 60.5 yards per game (27th in the NFL) and 2.63 yards per carry (29th).

"George does a lot for us, not just in the stats that you guys see in the yards and everything with catching the ball, but he is as big of a part of our run game as anyone also," coach Kyle Shanahan said. "Anytime you lose your best player, you need guys in both phases to step it up."

Beyond his impact as a blocker and pass-catcher, Kittle opens things up for teammates and gives the Niners an element of deception that creates confusion. Which is why it took more than one player to fill the void he left behind.

"You can't replace a guy that's arguably a top-five player in this league, regardless of position," right tackle Mike McGlinchey said. "It's impossible to do that."