The Nationals also put righty Koda Glover on the 60-day DL and selected the contract of catcher Miguel Montero on Tuesday, when Washington hosted the Minnesota Twins in the final exhibition game for both teams.
Murphy had surgery on his right knee shortly after last season ended and had been expected all along to miss the start of this season.
Benoit, 40, who joined the Nationals during spring training as a free agent, strained his right forearm. Glover has tendinitis in his right shoulder.
Montero gets a one-year contract that pays him $1.3 million while in the major leagues. He can earn $1.7 million in performance bonuses based on games: $200,000 each for 40, 50, 60 and 70, and $300,000 apiece for 80, 90 and 100.