This story is for CNBC Pro subscribers only. UBS floor director Art Cashin said Friday that the dip in markets this week had the potential to be a longer-term sell-off. "I think we're in a testing period here. The bulls are clearly on the defensive," Cashin said on " Squawk Alley ." The major U.S. indexes were on pace for their worst week since the market's most recent bottom in late March. Cashin said the week's trading had revealed some key levels of technical support. Investors should watch for support at the 22,900 level on the Dow, which is "absolutely critical," Cashin said. Stocks were down again on Friday after news that the U.S. was restricting the sale of semiconductors to Chinese telecom company Huawei. Cashin said that traders are worried that move is a sign of things to come in the relationship between the U.S. and China. "Traders are concerned not just about Huawei. They think that there's a chance that the President's team may adopt an anti-China and China's handling of the virus as part of the theme going into the election. And that will make for further trading bumps, and that will keep the bulls on the defensive for weeks if not months at a time," Cashin said. Cashin also discussed the plan to partially reopen the floor of the New York Stock Exchange later this month. Watch the full interview above.