NEW YORK ( TheStreet) -- The stock market will contend with multiple global economic concerns next week, but Standard & Poor's late Friday downgrade of the U.S.'s credit rating likely will top the list.

The ratings agency cut the U.S.'s long-term sovereign credit rating to double-A-plus from triple-A, the first time the U.S. has had its rating downgraded.

S&P said it made the decision because the recent debt plan approved by U.S. politicians isn't sufficient to "stabilize the government's medium-term debt dynamics."

The agency added that the downgrade broadly reflected that "the effectiveness, stability, and predictability of American policymaking and political institutions have weakened at a time of ongoing fiscal and economic challenges to a degree more than we envisioned when we assigned a negative outlook to the rating on April 18, 2011."

How the markets react to the news Monday is a matter of debate. Some investors have warned that pension funds with requirements to hold triple-A-rated assets will have to sell their Treasury holdings, which will cause bond yields to surge.

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But Marilyn Cohen, president of Envision Capital Management, says the market may have priced a downgrade in already and that the bond market will greet the downgrade news with a yawn.

"We're the deepest and the largest bond market around," she says. "Investors feel we're money-good, there's no question about that. As long as you have the illusion that you're money-good while you're borrowing 40 cents on the dollar, nothing will change."

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Stocks, which retreated from an early afternoon rally Friday as rumors swirled that a downgrade was coming, already may have factored at least some of the news into prices, others said.

"We could wake up on Monday morning and see the futures down precipitously, but I want to believe some of this was priced into the market," says Matthew Rubin, director of investment strategy with Neuberger Berman.

Before word of the downgrade hit markets Friday, investors were already on heightened global watch as they faced the coming week.

Growing concerns about the state of the U.S. economy boiled over this past week, helping to send all three major U.S. equity indices down at least 5%, as Wall Street pondered whether even the sluggish growth seen so far in 2011 can be sustained.

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