Stocks are in a "secular bull market" that's going to last many more years, veteran portfolio manager Martin Sass told CNBC on Thursday. But there will be corrections along the way, he warned.
Investors should not try to trade the pullbacks "because one doesn't know when that's going to happen or when you get back," the M.D. Sass CEO said in a "Squawk Box" interview. The boutique investment firm has $6 billion under management.
If anything, buy into the dips, he said. "The big picture is stay invested in good, sound, undervalued companies and there will be a lot of money to be made over the next several years."
Sass said the dovish Janet Yellen—if confirmed for the top job at the Federal Reserve—would lead a push "around the world of central banks fighting deflation, stimulating growth, injecting continued monetary accommodation that allows ... an eight-year expansion, at least, in the economy and the stock market."
One of the triggers that could lead to a correction: the Fed starting to scale back its $85-billion-a-month bond-buying program. "If I had to put a bet on it, I'd say March is the meeting they'd start the tapering," Sass predicted.
Jack Ablin, chief investment officer at BMO Private Bank, said on CNBC that the stock market is "fairly valued" based on the trillion-dollar injection from the Fed every year.
"I do think the market could drop say 10 percent or so once we get back to kind of the basic fundamentals," Ablin said. But he acknowledged, "Riding this liquidity and riding the momentum is not a bad place."
Stocks as measured by the broader-market have recently been trading near record highs again. So far this year, the index has increased nearly 23 percent.
—By CNBC's Matthew J. Belvedere. Follow him on Twitter @Matt_SquawkCNBC.