OWINGS MILLS, Md. -- If it feels like the Baltimore Ravens have allowed more big pass plays than they did all last season, you would be correct, much to the defense's embarrassment.

After struggling to contain the Cleveland Browns on Sunday, the Ravens have given up four pass plays of 50 or more yards in four games this season. That has already doubled Baltimore's total (two) in 16 games last season.

"We’re not a defense that gives up big pass plays. We just never have been, not consistently," coach John Harbaugh said. "And we’re not going to be that this year, and it’s going to get fixed. It should have been fixed last week. ... And I’m sure we’re going to get tested. I guarantee you Pittsburgh is going to be throwing those same types of routes, and we have to play them better."

A year ago, the Ravens boasted the No. 1 defense in the NFL. Now, Baltimore is No. 1 in the league in pass plays of 50-plus yards allowed.

In the Ravens' 40-25 loss on Sunday, Browns wide receiver Jarvis Landry turned a short pass into a 65-yard gain in the second quarter by breaking arm tackles from Marlon Humphrey and Patrick Onwuasor and bouncing off Tony Jefferson. It would've been an 87-yard touchdown if Landry hadn't tripped over his own feet.

A quarter later, Humphrey and Jefferson both jumped a hitch route by Odell Beckham Jr. and left tight end Ricky Seals-Jones uncovered for a 59-yard reception. How open was Seals-Jones? He had 8.8 yards of separation on the nearest Ravens defender when he caught the ball, the most of any Baker Mayfield's 31 career touchdown passes, according to NextGen Stats.

Has the defense's confidence been shaken by all these big plays?

"Doesn’t shake my fundamental belief, but it’s frustrating as hell," safety Earl Thomas said. "It’s like you take another step back after you put in work, talked about certain situations, you made corrections and when we get out there and they still have success, it’s very frustrating."

Here are three reasons why the Ravens pass defense is struggling:

Lack of discipline: Harbaugh believes the biggest problem is defenders not playing coverages the way they were designed.

"We’re overplaying some of our zone coverages," he said. "We’re overplaying some routes. We’re chasing some patterns that really fall in the category of trying to do too much. That’s not what we need to do. You just need to do your job, be in your right spot, and play football."

Baltimore had so many lapses that it allowed Mayfield to break out of an early season rut. According to NextGen Stats, Mayfield averaged 17.4 yards per dropback in Week 4 against the Ravens when facing zone coverage, the highest such rate of the week. In the first three weeks of the season, Mayfield averaged 4.9 yards against zone defense, which was the second worst in the league.

Injuries: Humphrey shadowed Beckham all game, holding the three-time Pro Bowl receiver to career lows in catches (two) and receiving yards (20). It's the supporting cast that has faltered after the injuries to cornerback Jimmy Smith (knee) and nickelback Tavon Young (neck).

The Browns capitalized on the absence of Young, who is out for the season. Landry recorded 154 of his 167 yards receiving Sunday when lined up in the slot, averaging 3.9 yards of separation, according to NextGen.

Free safety: The biggest change made in the Ravens secondary this offseason was Baltimore cutting Eric Weddle and replacing him with Thomas, who signed a four-year, $55 million contract that included $32 million guaranteed.

Weddle was described by defensive coordinator Don "Wink" Martindale as a "football savant" for how he confuses quarterbacks by disguising coverages. Thomas made the point in spring workouts that the biggest challenge was learning the Ravens' complex defense after playing Cover-3 all the time in Seattle.

Thomas entered this season as one of four safeties to record over 10 interceptions, 200 tackles and multiple touchdowns since 2015. In four games, Thomas has 15 tackles but has produced one interception and one pass breakup.

"It's a different defense, but it's still football," Harbaugh said. "He's doing a good job. He's going to come up with some plays. He made eight tackles [Sunday], and he could have had a few more that he's mad about. So, I'm pleased with where he's at, and to me, he's only going to get better in the system as he gets a better feel for it."