A "heartbroken" Jack Bird has vowed to bounce back from suffering a second ACL tear to his left knee after a freak non-contact injury at Broncos training on Wednesday.

Bird took to Instagram on Wednesday night to say that "it’s like my whole world has come crumbling down" after scans confirmed his worst fears.

The 24-year-old said he started his day on Wednesday "confident, happy and full of excitement" about the looming clash with the Cowboys on Friday night.

Those hopes soon came crashing down.

"People who know me know how hard I have worked to get to the stage I was at leading into this season, and with that, I’m truly heartbroken that I have to start this process all over again," he wrote.

"I understand that a lot of fans are upset with me over the last few seasons but injuries are definitely something I can’t control. I pray that you guys stick by me through this tough period of my life.

"If there is one thing you don’t know about me it’s that I will never give up and I am resilient like no other. I know for a fact that I will be back. I love you all, see you next season with a bang."

The Broncos released a statement to confirm the tear and Bird will now see a specialist in the coming days with surgery the most likely option.

Bird's injury occurred when he was about to be tackled by reserve halfback Tom Dearden. His left knee then buckled beneath him and he reached for it immediately.

"Rugby league is a cruel game sometimes," coach Anthony Seibold said.

"Jack’s commitment to his rehabilitation has been unwavering in his return from his last knee injury.

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"He has had a tremendous pre-season and ticked all the boxes required for a great start to the year.

"The club will do our best to help Jack in his recovery and personal well-being and provide him with everything he needs."

The Cowboys clash was set to be Bird's first Telstra Premiership outing at fullback for Brisbane after winning the battle with Jamayne Isaako for the No.1 jersey.

Isaako had been named on the bench and is set to come into the starting side.

Bird, who won a premiership with the Sharks in 2016, has won many admirers with the resilient and spirited way he has coped with the injuries that have plagued his three seasons at Brisbane.

He was on the way back from shoulder surgery in 2018 when complications from a sternum injury limited him to just eight appearances for Brisbane. Last year he played just nine games before rupturing his ACL.

On Wednesday morning Seibold spoke to the media before the Broncos trained and the last question he was asked was about Bird's stellar pre-season and his value to the team.

"In some ways he is like a new recruit for us. He was probably in our top two players last year in the first eight games and then he hurt his knee, and we missed him," Seibold said.

"He is someone who has won a premiership and played State of Origin football. What I like about Birdy is that he has been consistent and resilient over the rehab process. He has had a hell of a pre-season.

"I think he has got himself in good shape and performed particularly well in the trials. He has earned the jersey."

There is a poignancy about those words in the wake of what transpired minutes later but also hope that Bird can come back from his latest setback.