ST. LOUIS -- St. Louis Rams defensive tackle Aaron Donald has high standards.

"I haven't played my best football yet," the second-year player said Tuesday. "I still have a lot more I need to learn. I broke down film a lot during the offseason and saw a lot of things I didn't like. I did good things, but I had a lot of things I didn't do right. I have to fix that. I'm trying to play a lot better than what I did last year and keep improving."

What Donald did last year was establish himself as one of the best defensive tackles in the NFL.

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The No. 13 overall pick out of Pittsburgh, Donald had 72 tackles, 38 quarterback pressures, 17 tackles for loss, nine sacks and one forced fumble.

Donald became the first Rams rookie to be selected for the Pro Bowl since running back Jerome Bettis in 1993, and was The Associated Press defensive rookie of the year.

But the 24-year-old lineman is far from satisfied.

"It's just the beginning," Donald said. "I feel like I haven't played my best football. I feel like I had good games, but I didn't have that game yet that I can go back and watch and be like, 'I made it.' I feel like I had good games, but I didn't have that one game to solidify me as one of those top players yet. So I have to keep working my butt off."

Donald's attitude and work ethic have impressed the Rams.

"He's worked very, very hard the entire offseason," coach Jeff Fisher said. "All of the things he accomplished last year, he started this offseason on a mission. He's in great shape. He has a good feel. He knows every snap he has a chance to win, and he's motivated by that."

Donald said he worked all offseason to get better and got bigger, stronger and faster than he has ever been. Listed at 285 pounds a year ago, he said he reported for camp at 290 pounds but has lost body fat.

"I really believe he just scratched the surface last year," Rams defensive coordinator Gregg Williams said. "His understanding of the pro game, his understanding of how we want things done, and some of the freedoms that we allow him to do, to play around him. He's a rare person instinctive-wise. He's a rare person intelligence-wise about that position. And he's a rare person leverage-wise. He plays with great leverage all of the time."

Rams defensive end Robert Quinn said: "For a D-tackle that's so fast, so strong, quick twitch, all that stuff, you don't see that every year. To get a guy like that, it's special. There's no ceiling for him. As long as he keeps progressing and getting better and better, he can write his own story."

The Rams have amassed a wealth of talent along their defensive line over the past few years, and that group could be one of the best units in the NFL this season.

The defensive front is led by Donald and Quinn, a two-time Pro Bowler who has 40 sacks over the past three seasons, and includes five former first-round picks. Defensive end Chris Long, the No. 2 overall pick back in 2008, has 51 1/2 career sacks. Defensive tackle Michael Brockers was the Rams' first-round pick in 2012. Free-agent signee Nick Fairley was Detroit's first-rounder in 2011, going one pick before the Rams took Quinn.

Donald will certainly face more attention from defenses this season, but he's confident that if opponents want to double-team him, he knows that will lead to one-on-one opportunities for his teammates.

That said, the defensive tackle has worked to improve on his outstanding rookie campaign and expects to do so.

"I feel a lot better in this camp," Donald said. "I feel like I'm having a better camp than I did last year. I feel more comfortable now coming into my second year and knowing the playbook and knowing what Coach Gregg (Williams) wants us to do and how he wants us to attack. I feel a lot more comfortable and I feel like I'm improving, so that's good."

Notes: St. Louis Blues coach Ken Hitchcock visited practice with his assistant coaches. ... RB Tre Mason and CB Trumaine Johnson returned to practice after sitting out recently. ... CB Janoris Jenkins and DE Chris Long did not practice.