The Nationals placed Poulin on the 15-day injured list Sunday due to a left flexor strain.
Poulin hadn't pitched in either of the Nationals' first two games following the All-Star break, so it's unclear when or how he sustained the forearm strain. He'll be eligible for activation July 31, but given the nature of his injury, Poulin seems likely to miss more than the minimum 15 days. Washington re-signed left-hander Matt Krook to a one-year deal Sunday to take Poulin's spot in the bullpen and on the 26-man active roster.