The Brisbane Broncos are reportedly set to pull an enormous surprise for their must-win Round 25 game against the St George Illawarra Dragons, with Tyson Gamble to replace Ezra Mam in the five-eighth role.

Gamble had the role locked down earlier in the season until he suffered an injury.

Taking over from Albert Kelly who started the season alongside Adam Reynolds in Round 6 after the former suffered a long-term injury, Gamble would hold the jersey through to the end of Round 12.

During that time, Mam made his debut in place of Reynolds, who suffered an injury of his own, but when Gamble - who had impressed to spark six wins on the trot - was injured, it was Mam who was called into the side.

Mama has since played every game at five-eighth for the Broncos, and while his attack has been impressive, his defence is a cause for concern, with 56 missed tackles in 13 appearances - including 18 in the last fortnight.

Given Mam hasn't had a try involvement in the last two weeks, the pressure growing on Kevin Walters to keep Brisbane in the eight has finally caused a surprise, with Channel 9's Adam Jackson reporting the switch was made at training on Monday.

While Gamble doesn't have the same attacking gifts as Mam, his defence is noted as a strong part of his game for a half, and his competitiveness has the Newcastle Knights sniffing around to bring him to the Hunter next year after Brisbane gave him permission to negotiate an early release from his contract.

That may make this move from Walters surprising given Gamble is unlikely to be at the Red Hill-based club next year, however, this is as must-win as it gets for the Broncos.

If they beat the Dragons in Wollongong, then they will rely on the Canberra Raiders to lose to the Wests Tigers on Sunday, or a for and against turnaround of 43 points to sneak into the final top eight, with the Raiders holding the inside running on the final spot, and the South Sydney Rabbitohs now uncatchable after their win over the North Queensland Cowboys on Saturday night.

Teams will be named for Round 25 at 4pm (AEST) on Tuesday.