Due to become a restricted free agent in July, Butler split last season between Nashville and the New Jersey Devils. He posted four goals and 11 points in 34 contests.
In 128 career games, the 26-year-old Butler has amassed 20 goals and 28 assists with Nashville, New Jersey and the Ottawa Senators.
The Massachusetts native has experienced success at several levels. He finished as a Hobey Baker finalist in 2009-10 after leading the NCAA with 29 goals, and earned AHL All-Star MVP honors prior to capturing a Calder Cup Championship with the Binghamton Senators in 2010-11.
Brennan, acquired in trade with Buffalo on March 15, appeared in 19 games with the Panthers, scoring two goals and seven assists. He is also scheduled to become a restricted free agent this summer.