After a positive open, stocks suffered a sharp intraday slide on Thursday that saw the Dow Jones industrial Average drop by more than 200 points. Now traders will have to closely watch the next few sessions to learn whether the five-year bull market might finally be coming to an end, according to UBS' director of floor operations, Art Cashin.
"I don't know about the bubble bursting, but we've done some serious technical damage," Cashin said on Thursday's episode of "Futures Now." "The next several days will be absolutely critical. If we see continued weakness, you will begin to hear people talking about having put the top in."
In explaining the weakness on Thursday, Cashin turned to rumors about the volatile situation in Ukraine.
"Early in the day, when we tried to rally, the S&P ran into some very strong resistance at 1,878, 1,874. We began to pull back very slightly from there. Then the rumor wires went hot with a lot of vague rumors about Ukraine and Crimea—rumors about statements that hostility was growing nearer, further rumor about possible shots being fired. Now, none of that was confirmed, but it was enough to spook the market," Cashin said.
(Read more: Marc Faber: Unchecked bull market leads to big declines)
On Sunday, a referendum vote is scheduled to be held in Crimea, in an apparent Russian attempt to make the Ukrainian republic a part of Russia. Cashin says the fast-moving situation could end up having a big impact on financial markets, especially if it leads to sanctions against Russia.
"If they have to wind up putting in heavy sanctions, Iranian-type sanctions on Russia—which I think is unlikely—that would have very negative financial implications around the world," Cashin said. "It would hurt global growth."
At the same time, the recent decline in copper is causing serious concerns about China.
Copper "is used as collateral in the shadow banking system, and if the value of copper goes down, people will begin to call in those loans, and that will be disruptive. What the market will fear most of all is a kind of contagion, the type of thing we saw in 2008, where people sold whatever they could sell, including their grandmother's necklace," Cashin said.
(Read more: Cashin: Watching for China's 'Bear Stearns moment')
Cashin says that in order to predict Friday's action, it will be important to note how the closes on Thursday.
"Friday, traditionally, is the best day of the week. It is the shorts who tend to cover. But I'm not sure that we'll see much short-covering" on Friday, Cashin said.
"We're going to go into the technicals. It looks like we have just broken some interesting and maybe critical support in the S&P, which was 1,848 to 1,851. So if we close below here, you might get the chart people coming in and saying we're vulnerable to further selling from now on."
—By CNBC's Alex Rosenberg. Follow him on Twitter: @CNBCAlex.
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