Rosie has walked about 4 batters per nine innings over the course of his entire ML career. That's what Reyes has done in the minors that has you so worried. Rosie is valuable when he is going because he strikes a ton of guys out. Aroldis Chapman has a comparable walk rate, and he's a stud.
You know who else mows guys down? Reyes. We are talking about a starting pitcher aged 20 who has struck out 61 guys in 41 innings in AAA this year. He had great K rates at lower levels, too. We are talking about a pitcher whose minor league stats are comparable to Rosie except he is starting and pitching many more innings. Oh, and he's six years younger, too.
Rosie, at his peak, is a very good reliever for 75 innings. Reyes has the ability to grow into a Cy Young caliber pitcher for three times as many innings per year. There's no comparing their long term value. Rosie can almost certainly be a better Cardinal in 2016 if he turns it around, but Reyes's ceiling for 2018-2020 is much, much higher.
If the Cards offered Reyes for the closer of any ML team, you could count the number of teams who say no on one hand. It might even be zero when you account for the collective bargaining agreement making young talent more and more valuable.
