So he stopped trying.
"We (CU's defensive staff) were going gaga on getting the guy," said Brown, the Buffaloes' secondary coach. "We'd seen enough of him on the scout as a freshman (at tailback) to know about his size, speed and toughness.
"He was a tremendous competitor, and when I found out he wanted to make the change, well, yeah, I felt fortunate. I told him, 'If you want to relinquish that role (on offense), and as long as 'Hawk' (head coach Dan Hawkins) and 'Hags' (running backs coach Darian Hagan) were OK with it, then come on."
And come on Polk has, methodically moving into Brown's starting unit at strong safety. A still-green redshirt freshman, Polk is absorbing as much as Brown and the veteran players around him - senior corner Cha'pelle Brown, senior free safety Ben Burney, junior corner Jimmy Smith - can offer.
But he's proving to be a quick study, and his learning on the fly is benefitting the Buffs. At Kansas State last weekend, Polk registered 15 total tackles (nine solo, six assists), the highest total for a CU defensive back since former safety Ryan Walters made 16 against Iowa State last season.
That's a good news/bad news scenario for a defensive unit.
The good news: Polk is not only finding himself, he's asserting himself in the secondary. After making his second start of the season and playing all 60 snaps against K-State, he's scheduled to open Saturday against Missouri at Folsom Field (11:30 a.m., FSN).
The bad news? Let Hawkins deliver it, then expound on it: "You probably don't want your safety being your leading tackler, but he's doing a nice job back there. He's playing well and continues to improve. It's been a long journey for him, with the shoulders and coming from running back."
"The shoulders" is a reference to the surgeries Polk had last fall and spring, the latter operation sidelining him for spring drills and giving Brown some slight pause about how much ground Polk might be able to make up come August and into fall practice.
But Polk, a 6-foot-1, 210-pounder from Scottsdale, Ariz., who was ranked by one recruiting service as the nation's No. 11 running back in 2007, closed fast in his rehabilitation and in making the offense-to-defense transition.
Fact is, "closing fast" is a trait that accelerated his move into the starting lineup.
"The thing is, he's got so much speed he can run down guys when mistakes are made," said Brown, who spent 15 seasons in the NFL between his first and second stints at CU. "His tremendous speed allows him to make plays that other players couldn't make . . .
"He really brings the wood and sticks his face in there. He's most likely the fastest guy on the team; if he's not, I want to see the race. It'll be a heck of a contest."
Polk, whose father Raymond was a cornerback at Oklahoma State and was drafted by the Los Angeles Raiders in 1985, isn't into puffery about which Buff might be the fastest. He just knows he's fast enough in his new role to be effective.
"As a runner, I'm more of a straight-away speed guy, and that helps in the back half (of the defense)," he said. "I think it's the best position for me in my career and for the football team. It feels natural to me."
Polk came to CU in the same recruiting class as tailbacks Darrell Scott and Rodney "Speedy" Stewart. Had he not switched to defense, he has no doubt he could have competed with that pair, as well as Demetrius Sumler and Brian Lockridge, for carries this season.
"I think I would have gotten the chance to run the ball this season, but this has been nothing but good," Polk said. "I have no regrets at all. I'm just happy to be able to play and contribute."
CU's safeties call out on-field defensive checks before the ball is snapped, and Brown said Polk has adapted well to that responsibility.
"All those guys have shared the job, and I've put it on Ray, too," Brown said. "He's vocal and not hesitant at all. I'm very pleased with his progress. He's still learning in coverage, but everything for Ray is all there in front of him.
"As fast as he's coming along week by week, he's going to be a bona fide guy in the lineup for a long time to come."
Polk believes he's "pretty much adjusted" to making calls, but concedes, "I'm certainly not the best at it. I keep working every day; you've got to know everything that's going on.
"Now, I'm getting down to mastering the position - but that'll take a while."
His speed and physicality have been of great benefit in run support, and Kansas State's emphasis on the run last week pushed him to the forefront.
Missouri, though, will be a test of a different sort. The Tigers' offense figures to be more balanced, and Polk calls it "a tricky offense to know."
It was tricky enough over the past two seasons under former quarterback Chase Daniel to pummel the Buffs for 113 points, including a humiliating 58-0 loss in 2008.
"It was embarrassing last year; we don't want that to ever happen again," Polk said. "We owe it to Missouri and our fans to stick it to this team."
If that happens, it could signal the Buffs are serious about working their way into contention in the Big 12 Conference's North Division. CU, Kansas and Nebraska are 1-2 in conference play, Mizzou is 0-3. Kansas State (3-1) and Iowa State (2-2) are the front-runners.
As jumbled as the North is, and with no dominant team, CU (2-5 overall) still controls its fate. With five games remaining (three at home, two on the road), the Buffs need four wins to reach six for bowl eligibility.
In the wake of the 20-6 loss at K-State, CU's senior leaders called a players-only meeting Monday. Polk indicated the meeting was one of self-examination and re-dedication.
"We understood that we need to get it done . . . to come together as a team and just get it done, honestly," he said. "We're running out of season."