Jake Butt, the once-promising tight end whose football career was waylaid by six knee operations, announced his retirement from the NFL on Thursday, saying he could no longer hide the fact that he'd lost his passion for the game he loves.

"Football gave me some of the best times of my life," Butt wrote in a social media post announcing his decision to step away. "Ironically, it also gave me some of my toughest times, which turned out to be the most important. The adversity that I faced in my career gave me some of the biggest opportunities to grow as a man."

Butt had signed a one-year deal with the Chicago Bears last month after four star-crossed seasons with the Denver Broncos.

Butt "showed remarkable perseverance, dedication & mental toughness to overcome three ACL injuries as a player -- an incredibly rare accomplishment," tweeted Broncos chief communications officer Patrick Smyth. "His positive attitude & work-ethic thru adversity set a great example."

Butt entered camp this week competing with J.P. Holtz, Jesper Horsted and Scooter Harrington. The Bears also signed veteran tight end Jesse James to a one-year deal last weekend.

Butt first tore an ACL during spring drills his sophomore season at Michigan and he tore the other one in his final game for the Wolverines in the Orange Bowl. The Broncos drafted him three months later, but he spent his rookie year on IR.

In 2018, he played in three games before suffering a third torn ACL, this time at practice, and in 2019 his comeback was interrupted by yet another operation to clean out a torn meniscus.

Last year, Butt joined Washington quarterback Alex Smith as one of the NFL's biggest feel-good stories on cut-down day, overcoming his sixth knee operation to stand out among a deep group of Broncos tight ends featuring 2019 first-round pick Noah Fant and heralded rookie Albert Okwuegbunam.

But he was soon lost in the mix, playing in just five games with one start and catching two passes for 5 yards.

Once considered a possible vanguard to the heralded 2017 draft class of talented tight ends that featured O.J. Howard, Evan Engram, David Njoku and George Kittle, Butt finished his career with just 10 receptions for 90 yards and no touchdowns in eight games, four of them starts.

Still, his perseverance and tough luck made him a fan favorite and a locker room beacon. Quarterback Drew Lock declared last summer, "There's nobody in our locker room that doesn't want to see Jake Butt succeed."

At Broncos training camp last year, Butt said he'd overcome some dark days and the desire to walk away from the game after his latest knee operation.

"Bad days are going to come and tough days are going to come, but you can't quit," he said at the time. "You just have to keep showing up."

That desire no longer burns so hot.

"This may come off as impulsive," Butt wrote in his farewell. "Some may wonder, 'Why now?' when I was so close. But over time I've lost the passion that I once had for this game. I've battled through this feeling for a while now, but I can no longer ignore it.

"I will always cherish and love this game. It has given me some of my best memories, highest highs, my strongest friendships and my best lessons. But it's time for me to close this chapter and move on to a new one."

What that is, he didn't say.

Butt thanked all those who supported him in realizing his dream to play professional football and said he was "extremely lucky to have been led by such great coaches and to have shared the field with such amazing teammates."

At long last, Butt said, he's finally among football's fortunate.

"I am one of the lucky ones," he wrote, "who gets to step away healthy, on his own terms. And with a newfound excitement for this next chapter in my life."