TEMPE, Ariz. -- Bruce Arians noticed it about six weeks ago.

Arizona Cardinals first-round pick Robert Nkemdiche started practicing smarter, started showing less on the mental error sheet and started picking up on the small nuances he'd been missing during his first six months as a professional football player.

He started acting like a pro.

It led to Nkemdiche being active Sunday against the New Orleans Saints for the first time since Week 8, and has positioned him to finish the season on the field instead of on the sideline. Of 14 games this season, Nkemdiche has played in just four.

From the 10,000-foot view, Nkemdiche's rookie season has been a disappointment. A top-five talent who dropped to Arizona at No. 29 because of an incident at an Atlanta hotel last December, Nkemdiche hasn't put up the stats of a first-rounder. He has just two tackles.

But from the 1,000-foot perspective, this season has been one long lesson for Nkemdiche.

Robert Nkemdiche's work in practice is getting him more time on the field in games. Christian Petersen/Getty Images

"Over time I just kind of understood the ways of being a pro watching people like Calais [Campbell], Pat [Peterson], Larry Fitzgerald, and the people who were thriving -- Tony [Jefferson], Ty [Mathieu] -- just how to be a pro and what the coaches want from me," Nkemdiche told ESPN. "The trust issues. The little things.

"That's what I've been paying attention to and trying to focus on more and more every week."

Last week was one of Nkemdiche's best weeks of practice this season, Arians said. He's stopped making the mental errors that would have him lined up in the wrong gaps. He hasn't made a mental error in practice in a "couple" of weeks, Arians said. It translated to Nkemdiche playing 25 snaps in the loss to the Saints. While he had just one tackle, Arians felt Nkemdiche was "explosive."

"He's been practicing real hard, too," Arians said. "He should keep his numbers up.

"He's proving he's ready to play."

Nkemdiche has two weeks left in his rookie season to play up to the lofty expectations that preceded his arrival in Arizona. When Nkemdiche was in mid-sentence Wednesday, outside linebacker Markus Golden walked by and said, "He's going to be great."

Nkemdiche's finally in a position of to reach those heights. It just took him a while.

Coming out of Ole Miss, Nkemdiche didn't have an understanding of what the expectations were of an NFL player. His perspective on the details was different.

"I just didn't know the standards that things were held to," he said. "I didn't understand that the little things mattered as much as they did. It's a business and the margin of error is a lot (smaller) than it is in college. I didn't understand. It took me some time to get that understanding.

"(I need to) just keep evolving as a person and keep trying to get better as a player. That's what I try to do every day -- just learn from the older guys and just keep being humble, and just keep going forward."

Not understanding how to be a pro cost him this season. Arians' critical assault on the rookie began after a Week 1 loss to the New England Patriots, when Arians described Nkemdiche's play as "poor" and said "he was not a factor." A week later, Arians was critical of Nkemdiche's week of practice.

After a win over the New York Jets in Week 6, Arians was disapproving of Nkemdiche's effort, saying Nkemdiche didn't "have to be Superman just because you're a first-round pick." Arians continued to be hypercritical of Nkemdiche in public in early November, when he said Nkemdiche needed to develop a better sense of urgency in the weight room, meetings and on the field. And around Thanksgiving, Arians called Nkemdiche's maturity into question.

Nkemdiche could've calmed the criticism had he performed to the level of a top-five talent, but ankle injuries set him back in training camp and again early in the season. By time he recovered enough to play at full strength, he had lost his spot in the defensive line rotation and couldn't regain it.

But both his off- and on-field performances appear to be stabilizing.

"I just think he's more focused now," Golden said. "People got to remember he's only a rookie, too. No matter if you're a first-round pick or a top-10 pick, you never know. You still got to give him that time to learn. Give him that time to be a rookie and then you can judge him after that."

Golden has been one among the teammates who have counseled Nkemdiche this season. In return, Nkemdiche has asked "so many questions I can't even remember." Golden's perspective on being a rookie was among the freshest in the locker room having been drafted in 2015. One of the toughest parts of being a rookie in the NFL, Golden said, was adjusting to "something new." Rookies, Golden said, have to re-learn everything from the daily schedule on up.

The days of a first-round pick playing immediately on a good are all but over, Arians said. There's more of a learning curve, which was evident with Nkemdiche.

"He's learned what professional football is all about," Arians said. "You're a highly-recruited guy and you go into the SEC as one of the top players, you don't necessarily have to be a pro or work hard because you're more talented than everybody. When you get here, you're just another guy so you have to learn all that, and it's a shock to some of them.

"They have to learn to be a pro on and off the field."

It's not a new concept, Arians said. He's dealt with the same issues throughout his career but said it's happening more often in recent years because "society has changed."

But Nkemdiche's steps are evident, especially by how Arians talked about him this week.

It might have taken him six months to figure out life in the NFL, but Nkemdiche appears to have figured out the small stuff, which has added up in big ways for the first-rounder.

"Just on the field and off the field, just doing things that are going to make coaches trust you, just doing things right," Nkemdiche said. "Just being a role model. Just understanding that eyes are always on you and it can pay dividends by doing the little things right and just being a pro all the way around."