The team later confirmed the deal.
The 25-year-old inside linebacker started in 12 games in 2013 and came on strong down the stretch. He finished the season with 84 tackles, four passes defensed and an interception.
"He was the one who was calling the defense over there," McCoy told U-T San Diego. "He was the voice of the defense on game day. He did a great job, finished strong. Unfortunately, he got the (groin) injury earlier in the season, but I think he did a great job with his leadership."
Butler still has room to improve in coordinator John Pagano's defense. Locking him up gives the Chargers the chance to develop their former third-round pick.