U.S. credit woes may not be over, thanks to $4.2 trillion of dubious debt that comes due over the next five years, says Caitlin Long, head of corporate strategy for Morgan Stanley.

That debt is based in the commercial real estate and non-investment grade (junk) markets.

“At best, this wall of maturing U.S. debt will strain credit capacity,” Long writes in the Financial Times.

“At worst, it will prolong the credit crunch and restrain economic growth.”

The next two years will be telling, because delays by banks and other lenders in recognizing losses on commercial real estate loans could lead to a pile-up of debt maturities in 2012, she says.

“This is when loans to highly leveraged corporate borrowers begin to mature en masse. Such a 2012 reclogging of the credit system, if it happens, could force businesses to liquidate bad investments or pressure the Fed to re-open the monetary and credit spigots.”

The biggest risk would be if the Federal Reserve has to raise rates to control inflation and protect a sliding dollar, Long writes.

“Higher interest rates would preclude marginal borrowers from qualifying for refinancing, regardless of whether credit capacity exists.”

Not everyone shares Long’s concern about commercial real estate.

Joe Rodriguez, manager of Morningstar’s five-star-rated AIM Select Real Estate Fund, told CNBC that depressed prices in that market provide a buying opportunity.