Hagel has also spent time in Pakistan and co-chaired a 2009 Atlantic Council report with Sen. John Kerry that concluded that Pakistan faced, "dire economic and security threats that threaten both the existence of Pakistan as a democratic and stable state and the region as a whole."

"The U.S. also needs to urgently close the "Trust Deficit" between it and Pakistan, with greater exchanges of high-level visits, closer military, intelligence, and economic cooperation," according to the report.

The most immediate issue Hagel would face would be the future of the Pentagon's budget.

Just days before the United States reaches the edge of the "fiscal cliff" in which the Defense Department faces the loss of $500 billion on top of an already planned $500 billion in cuts, Hagel believes the Pentagon's budget is too big.

"The Defense Department, I think in many ways, has been bloated," Hagel said in a September interview with the Financial Times. "So I think the Pentagon needs to be pared down."

If Hagel becomes secretary of defense, he would have to have "great lines of communication" with members of Congress, who will oversee the Pentagon's smaller budget, Cohen said.