Gen. John Allen, the top U.S. commander in Afghanistan, "has worked on several options that we are now reviewing and working with the White House on," Panetta told reporters early Tuesday morning en route to diplomatic visit to Australia.
The Obama administration has already pulled the 32,000 U.S. troops attached to the Pentagon's 2009 surge strategy from Afghanistan.
The plan being crafted by Allen and other U.S. commanders will determine how the rest of the remaining 68,000 Americans will be drawn down over the next two years.
That drawdown is part of the White House's endgame for Afghanistan, which is to have all combat troops out of the country by 2014.
The Allen plan will rely on "several options based on the missions that we have to address in the [postwar] period," Panetta told reporters.
Joint U.S.-Afghan counterterrorism operations, along with training and assistance for the Afghan National Security Forces, will be the main missions dominating that plan, he added.
In October, top NATO commanders agreed to remain alongside their American counterparts in Afghanistan after the White House's 2014 deadline.
The alliance's postwar mission in the country will mimic the American plan, focusing heavily on Afghan National Security Forces training into 2015 and beyond, a NATO spokesman said at the time.
That said, the Allen plan could run into GOP resistance on Capitol Hill, not because of the recommendations themselves, but due to the architect of those efforts.
Allen, who was slated to become the new head of U.S. forces in Europe, has become ensnared in the growing sex scandal centered around former CIA Director David Petraeus.
Petraeus stepped down as the nation's top spy last Friday after admitting to having an extramarital affair with his biographer, Paula Broadwell.
FBI agents uncovered the affair after launching an investigation into Petraeus's personal e-mail accounts, which revealed evidence of his relationship with Broadwell.
Last Tuesday, news reports surfaced that the FBI inquiry began after a Petraeus associate, Jill Kelley, reached out to federal agents after reportedly receiving threatening emails from Broadwell.
On Tuesday, the FBI disclosed that in its investigation of the Petraeus affair, it unearthed thousands of email messages detailing inappropriate communications between Kelley and Allen.
As a result, administration officials have put Allen's nomination as European Command chief on hold, according to a DOD statement issued early Tuesday morning.
The four-star general will remain in his current position as top U.S. commander in Afghanistan, while the Senate will move ahead to confirm his replacement, current Assistant Marine Corps Commandant Gen. Joseph Dunford.
Dunford's confirmation hearing before the Senate Armed Services Committee is scheduled for Thursday.