Brewers right-hander Shaun Marcum and Cardinals lefty reliever Marc Rzepczynski previously were with the Blue Jays.
Marcum, who will start Game 2 of the National League Championship Series on Monday, sees a bright future for Rzepczynski in St. Louis.
"He's good. He's real good," Marcum said. "I was surprised that Toronto traded him. I saw him mostly as a starter in Toronto. He was in the bullpen a little bit last season."
The Blue Jays dealt Rzepczynski to St. Louis as part of the Colby Rasmus trade on July 27.
The lefty made 23 total starts in Toronto in 2009-10.
Out of the bullpen, in 28 appearances since joining the Cardinals, he struck out 28 while walking 11 in 22 2/3 innings.
Marcum was with Toronto from 2005-10, before he was sent to the Brewers last December.
"He's a guy who is going to command the strike zone," Marcum said of Rzepczynski. "He's got a really good, late, heavy sinker. He's got a really good slider, too. But that sinker, I know he learned that from Scott Downs last year, using that sinker to get quick outs.
"He's kind of taken that next step, and in my opinion, he's going to be one of the best left-handed relievers in the game."