The third-placed Warriors drew level on competition points with Penrith in second place, but are now poised to lose another playmaker on the run to the NRL finals.
Metcalf clutched at his right hamstring after landing the conversion for the Warriors' third try and cut a devastated figure leaving the field in Brisbane on Saturday night.
One of the NRL's most injury-plagued players, Metcalf is set for scans to determine his return-to-play timeline.
"Obviously, he was very emotional at the start but he's come good in that sense," said Warriors coach Andrew Webster.
"I'm sure he's hurting but he's been a good teammate. At halftime, he joined the team, sat down with them. We don't know any more about the injury."
Overlooked for most of the season, Metcalf had been playing only his second game since April after starring in last week's defeat of the Panthers.
But the St George Illawarra recruit's dream of leaving the Warriors as a premiership winner is now in doubt, just when he appeared back in favour at the New Zealand club.
Minutes before Metcalf's new injury, Walsh was crunched in a hip-drop tackle that landed Warriors fullback Charnze Nicoll-Klokstad in the sin bin.
Walsh initially attempted to play on, but in his next involvement he threw an intercept that allowed winger Alofiana Khan-Pereira to streak away and score.
He left the field as Khan-Pereira's try put the Warriors up three scores and on the way to a fifth victory in six games.
Brisbane's finals hopes are well and truly over, but Walsh's injury may also impact his hopes of playing for Australia at the World Cup from October.
Broncos coach Michael Maguire was unsure of the injury's severity immediately after the Broncos' 12th loss from 13 starts.
The Broncos were never in Saturday's contest, but a furious Maguire said it was "garbage" to suggest his side could be struggling for motivation in the final month of a dismal season.
"The boys are playing for the Broncos. They've got a badge that they wear. That is not good enough," Maguire said.
"I normally keep my cool but we need to have a good, hard look at ourselves and get into the game the way we know we can, or we'll get exactly the same next week."
Earlier, the Warriors made a fast start and scored first as Jacob Laban grabbed Wayde Egan's pass and snuck outside Ezra Mam to the tryline.
Metcalf batted the ball on for five-eighth Chanel Harris-Tavita to step twice through traffic and nab the Warriors' second try as they took a 20-0 lead into halftime.
The Warriors' Maori stars electrified after the break in Indigenous Round, with Nicoll-Klokstad touching down from a kick and Dallin Watene-Zelezniak finishing acrobatically in the right corner.
The Warriors may need to call on Luke Hanson for a second NRL game next week, given fellow halfback options Te Maire Martin (hamstring) and Tanah Boyd (knee) are both out.
Another option is to shift Nicoll-Klokstad from fullback and into the halves until Martin returns in coming weeks.
