Pearce left the field inside the final five minutes, with Jamie Buhrer filling in at halfback and placing a kick that resulted in the match-winning try to winger Shaun Kenny-Dowall.
Brown indicated post-game scans were set to confirm the extent of the injury but said it didn't look good.
The Knights coach also cleared any confusion during the game by confirming Pearce was replaced by Luke Yates as a free interchange - with Yates able to return to the field after passing a HIA examination.
"He's obviously disappointed, he was enjoying his footy and loves playing," Brown said on Saturday night.
"We'll find out probably more tomorrow but it's certainly a pec problem which is going to be some sort of long-term issue, whether it's going to be 12 weeks or the season we won't know until tomorrow.
"It's obviously not a great thing; when anyone loses their halfback and one of their best players it's not great. But we'd like to think with the experience that some of the younger halves have got over the previous few years, we feel our squad's stronger, we can overcome these things.
"Whatever the result is I'm sure he'll still contribute to the club moving forward."
Brown said the team would remain upbeat despite Pearce's absence, with young halves Brock Lamb and Connor Watson in line to join Jack Cogger in the halves against the Manly Sea Eagles next week.
Watson returned to training last week and was a genuine chance of making an early return from a shoulder injury against the Tigers.
"It's always a challenge when you lose good players but if we're going to get to the stage that we want to as a club we may have to manufacture a few things – JB (Buhrer) went in there today and came up with a kick play that came off the back of some good hard work from the team," Brown said.
"Each time we go into a game you've got to believe you're a good chance of winning. When you do believe that you give yourself a good chance.
"We've got Connor coming back next week, Brock played reserve grade today, JB filled in today and did a good job in the last six or seven minutes, and came up with the kick.
"We've got to go into the Manly game full of confidence, that's what we've got to do."
The injury to Pearce soured an otherwise thrilling result for the Novocastrians, who were forced to come from behind in the dying stages to steal the win.
Brown was proud of his players' never-give-up attitude late in the game against the odds, and questioned a couple of decisions that went against his side.
"Skidsy (Kenny-Dowall) caught the ball clearly in the in-goal and Luke Yates makes a great tackle on Elijah Taylor and hits him with his head. Any time something is 50-50 and you're the underdog side, you just don't seem to get these calls.
"We're getting stiff calls quite consistently and have got to keep winning games for people to give us hope. We've certainly come a long way."