Henderson, who was speaking to the Reuters Autos Summit in Detroit, said that the credit markets have been effectively closed to corporate borrowers except those with the highest credit ratings, but said the turmoil in the U.S. financial sector could add to the pressure.
"I would say things have been difficult already," Henderson told Reuters. "Capital markets have been quite difficult, and this is just going to make it more so."
He added: "We're in for some rough waters here at least for this week if not the next couple of months."
As part of a plan announced in July, GM is targeting cost reductions of $10 billion, and up to $5 billion in a combination of asset sales and new borrowing to shore up its liquidity position between now and the end of 2009.
Henderson said GM, like other companies in restructuring, was already facing tough credit market conditions at a time when equity financing and private equity firms have also been in retreat.
"In terms of raising capital, you've got pretty much closed debt markets for anything other than triple-A-rated companies. You don't have that avenue available to you," Henderson said. "If you look at any company that's got business challenges, the markets are very difficult to deal with."
Henderson said the difficulty accessing capital markets made it more urgent for the U.S. Congress to approve funding that would clear the way for $25 billion in low-cost loans for the auto industry to fund plant retooling and other investment in fuel-saving technology.
The framework for that loan program was included in last year's energy law that established the first large increase in fuel economy targets since the auto standards were introduced more than 30 years ago.
Lawmakers must appropriate about $3.8 billion to cover default risk to issue the loans, and Congressional Democrats have said they would like to pass the legislation to do that by the end of the month.
Henderson said GM would continue with its own restructuring plan regardless of what Congress does with funding for the $25 billion loan program or the rules that govern how much of that total GM would be eligible to borrow.
"The actions in July are necessary to transform our business. We have to do those. It's all about self-help," Henderson said. "The ability to access financing to finance these projects certainly would help -- actually, it would help a lot in the short term -- but it wouldn't at all change the actions that we've announced."
Hummer sale work continues
Meanwhile, GM expects to give potential buyers financial information about Hummer in the next few weeks, and is simultaneously looking at other asset sales, Henderson said.
GM, which in June hired Citigroup to consider all strategic options for Hummer, has not officially put the iconic gas-guzzling brand on auction yet.
Even as GM executives have publicly said they have received indications of interest in the brand, likely buyers from India, China, Russia and the Middle East have denied interest.
A credit crunch that began last summer has severely limited access to borrowing money, especially for private equity players, making large deals very difficult.
"Private equity, which was a source of capital, has really curtailed its appetite with concerns about credit," GM's Fritz Henderson said at the Reuters Autos Summit in Detroit.
"Ultimately someone has to buy assets in order to sell them and there has been a dearth of buyers."
Still, Henderson said GM is not delaying the sale process of Hummer to wait out the downturn.
"I would like to reach a conclusion on this relatively promptly because taking a lot of time doesn't help."
Henderson said there was "no deliberate delay and that the auction had not yet begun because "doing standalone financials takes time."
Henderson said that while he would like to see a decision made on Hummer by the end of the year, he does not see the consummation of a Hummer sale before 2009.
Henderson said the automaker is also considering sales of other assets at this time.
Some analysts have said GM could sell its OnStar unit, which specializes in in-vehicle communications and safety technology. Henderson declined comment.
Problems in Europe, China
On another topic, GM sees signs of slower auto sales in Europe and China although there are also signs sales in the United States, the largest auto market, may have hit bottom over the summer, Henderson said.
There is more reason to expect that 2009 U.S. light vehicle sales will be near 14 million units, roughly flat from this year's depressed levels, he said.
But he said there were some early indications that the U.S. vehicle market seems it might have hit a bottom in July.
"It feels like all of the negative variables are factored into the market, are factored into the consumer," Henderson told Reuters. "Perhaps we have found the bottom ... in the U.S. I can't say the same thing for other markets."