Rosario, a mid-level prospect who throws in the low- to mid-90s, was selected by Baltimore in last December's Rule 5 Draft and ultimately returned to the Brewers. Posting a 5.83 ERA in 14 starts and two relief appearances for Class A advanced Brevard County, Rosario was later demoted to pitch exclusively in relief at Class A Wisconsin (as he last did in 2010), striking out 42 batters in 33 2/3 innings. He turns 22 on Sept. 30.
Herrera, who will join the Mets in time for Friday's series opener in Washington, is a veteran of 115 Major League appearances with the Reds and Brewers, but did not distinguish himself in a brief stint with Milwaukee earlier this season. Much more effective at Triple-A Nashville, Herrera posted a 1.48 ERA and .203 opponents' batting average in 29 games. He turns 27 next month.
"Rosario has a good arm," Brewers general manager Doug Melvin said. "And Herrera has pitched very well [at Nashville]. We had a short look with Herrera, but he was in a tough situation."
The Brewers promoted Herrera to the Majors for road series in Chicago and Boston, where he surrendered six hits and four earned runs in 1 2/3 innings.
Because Herrera was on Milwaukee's 40-man roster, the Mets technically claimed him off waivers from the Brewers and formally traded for Rosario.
Both Herrera and Rosario were on a list of five Minor Leaguers from which the Mets could select two as compensation in the July 12 deal for Rodriguez, who is 3-0 with a 3.12 ERA and 14 holds in his first 19 games with Milwaukee.