Ruggiano was injured trying to make a diving catch in the ninth inning of the Cubs' 7-5 loss to the Arizona Diamondbacks on Wednesday. With two outs and the score tied 5-5, Aaron Hill hit a fly ball down the right field line, and Ruggiano fell near the visitors' bullpen mound as the ball landed in fair territory. Two runs scored on the play, as the Cubs wasted a 5-2 lead in the ninth inning to lose on the 100th anniversary of Wrigley Field.
"About three steps before I went into a dive I felt a pull in the hamstring," Ruggiano said Thursday. "One step before I went into my dive I felt a pop."
Ruggiano said it's a Grade 2 strain and the best-case scenario is that he misses three to four weeks.
Ramirez, 24, was acquired in the Matt Garza trade with the Texas Rangers in July. He has a 7.71 ERA in six relief appearances at Iowa this season. He went 9-3 with a 3.68 ERA in 22 starts for Double-A Frisco and Double-A Tennessee.
"It's a dream come true," Ramirez said of his first call-up to the major leagues. "Everyone says that, but when you get that call it's definitely that feeling."
Rosscup returns to the Cubs for a second stint this season. He was the 26th player for the second game of a doubleheader against the New York Yankees on April 16 when he pitched 1 2/3 scoreless innings in relief.
The Cubs will play with 13 pitchers and 12 position players while Ruggiano recovers.
ESPNChicago.com's Jesse Rogers contributed to this report.