The quick rebound after this month's brutal stock-market rout is a cause for concern, Goldman says

• This month's sell-off was "a warning shot," Goldman's head of asset allocation research said.
• He said it's concerning how fast the market has recovered.

The stock market was quick to recover after a brutal sell-off earlier this month — and that's a reason to be cautious, according to Goldman Sachs's head of asset allocation research.

"What's concerning now is how quickly the market has gone back to where we were before, and we can discuss that, but certainly that shows that we are sadly nearly back to the same problem we were at a month ago," Christian Mueller-Glissmann said in an appearance on CNBC on Wednesday.

He called the sell-off "a bit like a warning shot," indicating the possibility for more volatility ahead.

The S&P 500 lost 3% on August 5, its biggest daily drop since 2022 amid an unwinding of the yen carry trade and worries of a US recession sparked by a weak July jobs report.

The market quickly pared those losses, however. The Dow Jones Industrial Average has risen more than 6% and the S&P 500 has climbed 8% since the sell-off, fueled by investors' confidence in a September rate cut and positive economic data that revived hopes for the economy to stick a soft landing.

But Mueller-Glissmann says investors shouldn't be so quick to let their guard down.

"Going into this, you had like one or two months where positioning and sentiment was at the upper end of the range. People were bullish," Mueller-Glissmann said.

At the time, he worried about the possibility of a correction because of weak macro momentum.

"You had negative US macro surprises for one and a half months before that, and you actually started to see Europe and China macro surprises turn negative as well," he said.

Now, the market seems to have rebounded, which Mueller-Glissmann says is understating risk, even if the sell-off on August 5 was "obviously a huge technical overreaction."

He says that while the market has bounced back, investor sentiment hasn't.

"What I would say is, the good news is while the S&P is back to where we were before, the complacency isn't. We're not at the same kind of extreme bullish sentiment and positioning," he said.

Other commentators have also noted that the sell-off may have been just a taste of more volatility to come.

Shortly after the market clawed back its losses, JPMorgan analysts said the rout was a 'dress rehearsal' for what's to come amid growth concerns, while LPL Financial's chief equity strategist says to expect a double-digit S&P 500 decline in the next few weeks.