ORCHARD PARK, N.Y. -- When New England Patriots quarterback Tom Brady watched film last week in preparation for Monday night's game, he learned what most Buffalo Bills opponents have realized this season: Cornerbacks Stephon Gilmore and Ronald Darby have become an obstacle -- and not an opportunity -- for offenses.

"It was a little disappointing to see from my study that they were playing as well as they were," Brady said. "But that's part of it. You're going to go against good players."

Just how good is the duo? Bills coach Rex Ryan coached Darrelle Revis and Antonio Cromartie -- who have 10 Pro Bowls between them -- for four seasons together with the New York Jets, and he believes Gilmore and Darby are on par with them, if not better.

"Those two are probably playing as good as I've ever seen, for two corners," Ryan said last week. "They are doing a tremendous job."

Darby and Gilmore have only two interceptions each, but they sit at or near the top of the charts in passes defensed. The NFL has credited Darby, a rookie the Bills selected in the second round, with an NFL-high 20 pass breakups. Gilmore is tied for fourth with 17.

Ronald Darby leads the NFL with 20 pass breakups. Brett Carlsen/Getty Images

Passes defensed can indicate a cornerback is being targeted more frequently by quarterbacks -- which isn't always a good sign. But former NFL safety Matt Bowen believes Darby is simply being targeted because he's a rookie, and he has proved to be up to the task.

"Those passes broken up, what that speaks to is a guy that's able to find the ball at the point of attack," Bowen, a contributor to ESPN Insider, said last week. "There's a lot of corners in the NFL -- especially young kids -- that when they get at the top of the route or at the catch point, they don't finish the play.

"But with Darby, he's very smooth at that point of attack, which is so impressive for a young kid. And I love watching him play."

Darby's position coach, veteran NFL defensive backs coach Donnie Henderson, didn't just discount any talk of Darby's unmatched number of passes defensed being the product of more frequent throws -- he questioned cornerbacks who believe they're better off when balls don't come their way.

"[To be thrown at is] even better," Henderson said last week. "That's what you want. 'If you throw at me, I'm winning.' Unlike some guys [who] don't get thrown at and they think they're winning. Well guess what? Who are you competing against? At least [Darby] is competing and winning."

Unprompted, Henderson brought up Darby as a candidate for NFL defensive rookie of the year. There were six cornerbacks taken before the Bills plucked Darby, from Florida State, off the board in the second round with the 50th overall pick. Of that group and the rest of the rookie cornerback class, only Kansas City's Marcus Peters (four interceptions, 17 passes defensed) and Green Bay's Damarious Randall (two interceptions, 13 passes defensed) come close to matching Darby's production so far.

Among rookies at other defensive positions, Saints linebacker Hau'oli Kikaha started off red-hot this season with four sacks and three forced fumbles in his first six games, but he has been hampered by an ankle injury in recent weeks. Jets defensive lineman Leonard Williams has played well but doesn't have the playmaking stats (0.5 sacks with no forced fumbles) to get him more attention.

Not only has Darby been one of the NFL's best rookies this season, he has been one of the NFL's best cornerbacks, period. Pro Football Focus' play-by-play grading of each NFL player has Darby receiving the league's third-highest score for a cornerback this season, behind only Carolina's Josh Norman and Arizona's Tyrann Mathieu, a safety who plays a lot of slot corner for the Cardinals.

"He is making plays," Gilmore said last week of Darby. "Even when he is not patient he has got the speed to make up. That is what makes him so good. But he is patient and he is making plays."

Gilmore isn't far behind: PFF has graded him as the ninth-best cornerback in the NFL, tied with the Giants' Dominique Rodgers-Cromartie.

The 10th overall pick by the Bills in 2012, Gilmore has started every game for which he has been fully healthy over the past four seasons. His performance this season, Henderson stresses, isn't anything new.

"Go back and look at his numbers last year," he said. "There were only four or five of them as good when it was all said and done."

Stephon Gilmore has been a mainstay in the secondary since he was drafted in 2012. Brett Carlsen/Getty Images

Gilmore has been able to tutor Darby, who was thrown into a starting role this summer after longtime starter Leodis McKelvin reinjured his broken ankle, landing the veteran on the reserve/non-football injury list to start the season.

Darby struggled this preseason. He allowed long catches by Carolina's Ted Ginn Jr. and Devin Funchess in the preseason opener, was beaten by Pittsburgh's Martavis Bryant in the third game and was flagged three times in August for pass interference.

Henderson explained that Gilmore took Darby "under his wings," teaching him how to defend the NFL's taller, bigger receivers.

"[I learned to stay] patient at the line," Darby said last week. "That is something I had a lot of trouble with coming out because in college I was just moving, giving a lot of ground, but [you learn to] trust in your ability, stay patient, try and get hands on the receiver."

Bowen recalled talking to an NFL scout last year who believed Darby had "natural coverage ability," and Bowen pointed to a pair of plays in the Bills' win earlier this month over the Jets as examples: a third-and-11 incompletion in the second quarter and a fourth-and-2 tackle for a loss in the fourth quarter.

"I think the Bills are playing blitz-man [coverage] and Darby's playing off [the line of scrimmage]," Bowen said. "He reads the slant and he breaks downhill and breaks it up. Beats the receiver [Jeremy Kerley] to the football. That's as good as it gets.

"Now also on Thursday night, how about the fourth-down play, when they throw it out there, a little smoke route? They say, 'We're gonna make the rookie tackle.' He comes up and makes the tackle."

Such plays are what makes Ryan's defense tick: Cornerbacks are placed in man coverage and relied upon to make plays.

"That's what a corner wants to do," Henderson said. "You know why? All of my concentration and focus is for this snap to whip this guy's tail. Most of them would love to do that. Go back to the old days in this game. That's what they played: man [coverage].

"And with the Rex defense, it gives you the opportunity to either be the hero or the goat."

Bowen, who said he played a similar scheme under defensive coordinator Gregg Williams with the Washington Redskins, was impressed with the physicality of Darby and Gilmore as early as their opening-day win over the Indianapolis Colts.

"Watching that film, I was saying to myself, they didn't give those Colts receivers room to breathe," he said. "They challenged them all over the field. And when you have that, that's a beautiful thing.

"That's a perfect situation for Rex. Now he can run his defense because he has those guys."