Piakura, 21, scored two tries that showcased his brilliant "hole-running" ability against an understrength Cowboys outfit in Mackay on Sunday.
He didn't play out the full game and was seen after the match wearing a knee brace.
Coach Kevin Walters said scans on Monday would determine the extent of the injury.
"He didn't take any further part in the game, so that is not great news for Brendan, or for us for that matter," Walters said.
"We will get some scans tomorrow when we get back to Brisbane. I am sure they will get on top of it."
Brendan Piakura will have scans after suffering an injury in the Broncos' win over the Cowboys. (Darren England/AAP PHOTOS)
Piakura is set to take the Broncos' starting second-rower position that was vacated by Kurt Capewell, who is now at the Warriors.
"Piakura is a very special player. He has great movement, great skill and can be a real X factor for the Broncos this season," former Broncos centre Brent Tate said in Fox League commentary.
Haas was put on report in the first half on Sunday for a swinging arm on Cowboys bench hooker Kyle Schneider.
The star 24-year-old prop's next scheduled match is the round-one clash with the Sydney Roosters in Las Vegas.
Haas, who ran for 125m in a blockbusting first-half display, has a nervous wait for the charge sheet from the match review committee, although there didn't appear much in the incident.
The Broncos got plenty out of the trial, which featured fullback Reece Walsh in electric form.
Veteran winger Corey Oates had a quality start to his season when he crossed in just the second minute.
His try came after Walsh chimed in and found new centre Selwyn Cobbo.
Cobbo's soft hands were a feature as he set up Oates to perfection.
Halfback Adam Reynolds owned the second try with a double pump and silky short ball for flying back-rower Piakura to race into a giant hole to score with ease.
Cowboys winger Semi Valemei swooped on a long Walsh pass to race away against the run of play, before Piakura scored his second try by showcasing classy late footwork to get himself outside the defenders just as he was about to receive the ball.
The unheralded Cowboys took it to the Broncos late in the first half.
Powerhouse back-rower Kulikefu Finefeuiaki carried three Broncos over the tryline with him, before hooker Sam Mullany charged through some poor Broncos defence.
After half-time the Broncos' big guns came to the fore, with Cobbo snaring a try from a bomb and Walsh scoring a cracker after an interchange of passing with Cobbo.
Last year's NRL grand finalists didn't look back from there, with the speedy Walsh setting up Tristan Sailor for a try.
Giant 205cm Broncos prop Ben Te Kura scored a try in an impressive late cameo appearance.
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