Munro scored two first-half tries against the Dolphins on Friday night but did not return after half-time as the Rabbitohs sealed a 16-14 win to open their NRL season.
At full-time, Munro was seen with his right arm in a sling and, after only one game to begin the year, told teammates he feared another collarbone injury.
A long-term injury would be a bitter blow for Munro, who managed only four games last season as a result of two separate collarbone fractures.
"He's done one in both shoulders now," Souths coach Wayne Bennett said at full-time on Friday.
Munro had been due to begin last season as a first-choice winger at Souths before his injury struggles, and looked poised to finally have extended time in the backline at first-grade level this yera.
The 20-year-old has already shown glimpses of his potential, and won over the rugby league community by scoring two tries against Penrith days after his mother's sudden death last August.
Stand-in Souths captain Cody Walker said the Rabbitohs were feeling Munro's pain after his latest injury.
"Everyone's disappointed at the club but he's already positive as it is," the five-eighth said.
"I was just talking to him in the sheds, he thinks he's done it again, but he said he'd be right."
The depleted Rabbitohs can ill-afford to lose more troops given Cameron Murray (Achilles), Latrell Mitchell (hamstring), Alex Johnston (Achilles) and Euan Aitken (pectoral) are already out.
Utility Jayden Sullivan came into the backline against the Dolphins after Munro went down but specialist winger Fletcher Myers appears likelier to start in Souths' round-two game against St George Illawarra.Â
As Munro began another stint in the casualty ward, Souths centre Campbell Graham and Dolphins captain Tom Gilbert made their long-awaited return on Friday night.
A troublesome sternum injury had sidelined Graham since September 2023, while Gilbert's shoulder and knee issues had combined to keep him out since that year's State of Origin series.
Between them, the two men had been absent from the NRL for a combined total of more than 1200 days.
Gilbert said it had been emotional to make his return after so long.
"I just wanted to embrace it," he said.
"You never know when your last game is. I just really enjoyed the week and saw the opportunity to play another first-grade game. I just wanted to take it with both hands."
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