"That's going to be his role, setting up, late-inning relief," Johnson said. "If he's on the ballclub, I'm using him. And he threw the ball good. One-two-three, can't beat that."
Rodriguez battled both wildness and injuries in the past. Last year, he issued nearly seven walks per nine innings and had season-ending elbow surgery in August, after starting out as Washington's closer and picking up nine saves.
The 26-year-old also dealt with right forearm tightness this spring and didn't secure a place on the 25-man roster until a strong outing in last Friday's exhibition game against the Yankees.
Rodriguez backed up that decision on Thursday. He induced a pair of routine outs from Juan Pierre and Placido Polanco and struck out slugger Giancarlo Stanton looking on a slider that started away and sliced over the plate.
Rodriguez's usual high-90s fastball topped out at 95 mph -- Johnson suggested the cold weather might have been a factor -- but the clean 12-pitch frame suggested he could be another weapon for the Nationals' loaded bullpen.
"He threw some good pitches," Johnson said. "He made progress all spring."
Washington's relief corps finished the three games against Miami with eight scoreless innings, allowing two hits and three walks while striking out eight. Last season, the bullpen ranked third in the National League in ERA, and in the offseason added closer Rafael Soriano, who saved the first two games.
Andrew Simon is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.