The Nationals enter the weekend needing reverse some serious negative momentum. Thursday night’s 6-2 loss to the Cardinals in St. Louis made for five straight. The Nats haven’t led in a game since late Sunday, a streak now at 38 innings—during which they’ve been outscored 23-10. They now trail the Mets by 7.5 games in the NL East. Last night, recent call-up Michael O’Conner made an emergency start as John Patterson nurses his sore…