"A commander in the field should never feel constrained from asking for what he needs," Gates told a House committee hearing.
Gen. David McKiernan was named to his post less than a year ago by former President George W. Bush and spent much of his tenure pleading for reinforcements for a backsliding war. Gates announced McKiernan's ouster on Monday, saying he wanted new leadership and thinking to match the new commander-in-chief's battle plan.
Troop increases for Afghanistan came slowly or not at all last year, in large part because so much of the Army and Marine Corps fighting force is tied up in Iraq. Although thousands of troops are now flowing in, they will come too late to make any difference for McKiernan.
Gates said the United States will have some 68,000 troops in Afghanistan by the end of the year, about double the total at the end of 2008. McKiernan has asked for about 10,000 more next year, and it is unclear what will happen to that request after Lt. Gen. Stanley McChrystal takes over.
Adm. Mike Mullen, the chairman of the Joint Chiefs of Staff, testified alongside Gates before the House Armed Services Committee. He said the number of additional forces already approved for Afghanistan is "about right," suggesting that any further additions would be small.
President Obama approved more than 21,000 new U.S. troops for Afghanistan this year as part of a wider strategy to confront the Taliban in select areas and protect the Afghan population from the militants. Although Obama campaigned on a pledge to close down the Iraq war and transfer resources to what he called the more important fight in Afghanistan, his military advisers have warned that troops alone are not the answer.
Gates has said the United States could never send enough troops to pacify and protect the sprawling country on its own, and must rely on the help of international partners including NATO in the short run and on the Afghans' own forces in the long term.
"We're certainly going to be there for awhile," Mullen told the committee. He added that he hopes this year and next will be turn the tide militarily.
"In the interim, we're going to have more casualties," he said.
As to McKiernan, Gates said he assumes the general "will retire with the honor and respect that he deserves."
McKiernan had expected to keep his job for about another year. Gates called his exit "an accelerated change of command."
"There was certainly no intent to convey anything negative or denigrate him in any way," Gates said.
Gates is the only member of Bush's Cabinet to be retained in his job under Obama.