Playing in his first match since rupturing his achilles tendon last August, Johnston was hurt in the ninth minute on Thursday when he landed awkwardly while scoring a try.
The 30-year-old then appeared to lack some confidence for the rest of the first half, and he did not return after halftime as Souths held on to win 28-18.
Rabbitohs officials believe Johnston has dome some damage to his left scapula, but are hopeful it is not serious ahead of next Friday's Sydney Roosters blockbuster.
"He's pretty good," Souths coach Wayne Bennett said.
"We're not sure whether he'll miss a game or a couple of games, but it's not on the too-serious list."
Ever the comedian, Johnston took to social media to laugh off the injury.
"Good to be back, it's been a while I forgot how to dive safely," he wrote on an Instagram story.
Johnston's fitness would come as a big boost for both the winger and South Sydney.
The Rabbitohs are already without centre Campbell Graham for around three more games, after he suffered a calf tear in last week's loss to Cronulla.
Johnston is also off contract this year, with negotiations yet to start.
Thursday's try got him to within 16 of equalling Ken Irvine's record mark of 212 career tries, with Bennett keen to see him break the record in Bunnies' colours.
"He's been a lifelong player here and is closing in on being the leading try-scorer in the game," Bennett said this week.
"The club would like to see him finish his career here.
"We'll talk to him ... We want him to stay, but they've got to want to stay as well and be happy with the deal."
Any Johnston absence could be somewhat offset by Latrell Mitchell's return, with the superstar fullback due to come back from a hamstring injury.
When Mitchell and lock Cameron Murray went down injured within minutes of each other at training last month, critics had Souths going close to the wooden spoon.
But they finish the opening month of the NRL with a 3-1 record, handed the base to contend for the finals after wins without their biggest-name players.
Bennett is unsure if he will be able to fit fill-in fullback Jye Gray into his first-choice 17 after impressing at fullback, but made clear what he wants from Mitchell in that position.
"We've won three out of four with a number of players missing. All Latrell has to do is come back and do his job for us," Bennett said.
"We don't want him to doing anything more than that. He's a wonderful player, we all know what he brings.
"But we need him to be in sync with the rest of us, working hard and being part of the team. I am sure he will do all that. I have no more expectation of him than that."
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