Greg Hardy and Allen Crowder battle it out until Hardy levels Crowder with an illegal knee, with the bout ending in a disqualification. (2:32)

NEW YORK -- Greg Hardy's first appearance in the UFC ended in controversy Saturday night due to a monster illegal strike at Barclays Center.

Hardy (3-1) suffered his first official mixed martial arts loss at 2:28 of the second round against Allen Crowder when referee Dan Miragliotta disqualified the former NFL star for an illegal knee. Hardy threw the knee as Crowder (9-4) clearly had one knee on the canvas, which is illegal.

As Miragliotta moved in to pause the heavyweight action, Hardy got in one follow-up right hand. Cageside doctors declared Crowder unfit to continue, and the bout was waved off, resulting in a victory for Crowder.

The final sequence is sure to spark plenty of debate. The rule against kneeing a downed opponent is well-known in the sport. However, Hardy is still very new to MMA, having made his professional debut only seven months ago. Hardy, 30, appeared to be exhausted at the time he threw the illegal strike.

"I was aware of the rule as it stands; I was not aware that that was what was going on," Hardy told ESPN. "I got deep into the fight, was in the moment, was trying to time it -- and I mistimed it. For that to end my day, man, is a hard pill to swallow, man. And it's a hard way to go down, especially when I wanted to portray a lot of appreciation and show everyone that I'm here to do it the right way, man.

"This is not how I wanted to do things. And I just hope everybody understands this is not who I am, that it's not something that I would do purposely."

Kneeing to head of grounded opponent A grounded fighter is defined as: Any part of the body, other than a single hand and soles of the feet, touching the fighting area floor. To be grounded, both hands palm/fist down and/or any other body part must be touching the fighting area floor.

A single knee, arm, makes the fighter grounded without having to have any other body part in touch with the fighting area floor.

At this time, kicks or knees to the head will not be allowed.

-- Via UFC rule book

UFC president Dana White attributed Hardy's illegal knee to inexperience in the Octagon, calling it "a rookie mistake."

"He was in his first real fight tonight. He's got three fights, he fought a guy with 13 fights, you know what I mean?" White told ESPN. "Greg Hardy can fight. He's got a chin -- he can fight. And he made a rookie mistake tonight, and it cost him a perfect record.

"He knows what he needs to work on. He knows what he needs to do and get back in the gym. And we'll see where we go from here."

Very few expected the heavyweight fight to last as long as it did.

A former All-Pro defensive lineman, Hardy had finished all three of his professional bouts by knockout in under one minute entering Saturday. The UFC signed him to a development deal last June, and it elected to put him into the Octagon relatively quickly.

Hardy landed several hard shots early in the fight, but Crowder, 29, managed to hang on and take him down. It was the first time Hardy ever had been taken down in his career. He did reasonably well defending himself, even working back to his feet. That effort, however, appeared to empty his gas tank.

Clearly feeling emboldened in the second round, Crowder began to taunt Hardy on the feet. He dropped his hands and talked to him, daring Hardy to hit him. Hardy remained calm and clipped Crowder with a couple of punches moments later. He also defended a deep takedown attempt, which led to the position of the illegal knee.

"I belong here, I belong on the main card, I belong in this community, in this organization," Hardy said. "And I will get better. I do take responsibility, and that's a big part of what I want everybody to know. That's why I belong here."