He based the rating on a review of the company's fundamentals, Squali told CNBC's "Halftime Report."
"Frankly, I would hate to be Twitter's CFO right now, looking at reporting earnings in about a month's time and having to justify valuation at this level and give new investors reason to pile up here," he added.
While Twitter remains a fast-growing companies, its valuation makes stocks such as Facebook and Google more attractive, said Squali, who does not own Twitter shares.
Josh Brown of Ritholtz Wealth Management took issue with the downgrade.
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"Why was it a buy rating before today? There's no consistency as to the timing other than, 'Hey, it's a new year. Let's reset expectations.' It's a smart political move by the analyst," he said. "I don't think it really carries much weight as far as a change of an opinion. [Twitter] has always been overvalued."
Brown said that he owns shares of Twitter.
"I hope it gets killed after they report. I'll double my position," he said. "I just don't think that it will."
OptionMonster's Jon Najarian still saw upside for Twitter.
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"I intend to hold it for a couple of months," he said. "Feb. 5 is going to loom large for my portfolio, as far as this earnings announcement."
Mike Murphy also remained a long-term bull on Twitter.
"I think there's going to come a time where people throw in the towel on Twitter when there's some sort of hiccup, and you're going to see a major hit in the stock," he said. "Because there is real growth there, there's a reason to own the company."
— By CNBC's Bruno J. Navarro. Follow him on Twitter