Rodriguez is a 6-foot-4 right-hander from Puerto Rico and a former eighth-round pick in the 2008 First-Year Player Draft. He spent last season with Double-A Reading, going 7-7 with a 4.23 ERA in 29 games (22 starts) and 136 strikeouts in 134 innings - placing him third in the Eastern League.
Rodriguez was chosen to the World roster for the MLB Futures Game and was named an Eastern League midseason All-Star.
In 2011, Rodriguez led the Florida State League in wins (16) and finished second in ERA, strikeouts and innings pitched, and was tied for second in starts.
Rodriguez is 31-20 with a 3.29 ERA and 494 strikeouts in 441 innings over five minor league seasons. He's expected to begin the year at Double-A Bowie.
"He's got a good arm, good stuff. Got a good breaking ball, and he's got a winning record since he signed. More strikeouts than innings pitched," said executive vice president Dan Duquette.
"He's got all the ingredients you look for in a young pitcher."
Rodriguez didn't rank among Baseball America's Top 30 Phillies prospects this year.
Welty, 25, is a career .284/.359/.468 hitter in the minors. He spent most of last season at Bowie.
The Orioles drafted Welty in the 20th round in 2008.
"They liked Welty," Duquette said. "Welty had a good year last year with the bat."