Wacha, who was moved to the 60-day DL to create space on the 40-man roster for Masterson, is not expected back until September.
"I think that's one of the hazards, if you will, of going with a very young rotation," Mozeliak said. "Putting this type of demand on 22-, 23- and 24-year-olds is tough. ... You look at how this rotation now sets up, it allows us to give some of the younger guys a breather and we can really space this out and manage better than we were doing."
Masterson, 29, will make his Cardinals debut on Saturday in a home game against division-leading Milwaukee. He will join the team a day earlier in St. Louis, at which time the Cardinals will make a corresponding 25-man roster move. He has been assigned No. 59.
No money exchanged hands in this deal, meaning the Cardinals will be responsible for the little more than $3 million remaining of Masterson's 2014 salary.
"I was surprised it took place, but I understand ... it's bittersweet, sad to leave the fellas and the fight that we're making here in Cleveland," Masterson said on Wednesday. "But [I'm] excited to get to St. Louis and be a part of what they have going on there."
Masterson's stock has dropped over the past year, as he went from a 2013 All-Star who was engaged in extension talks with the Indians to a pitcher who had not performed well enough to position himself for a qualifying offer from the club this winter. He will be a free agent in a few months.
Saturday's start will be Masterson's first since July 7, as he has spent the past three weeks on the disabled list resting an inflamed right knee. Masterson has pitched through knee discomfort for most of the season. Mozeliak had the organization's medical staff evaluate Masterson's medical history before finalizing the deal.
"Like any deal, you just want to make sure it's nothing that is going to prohibit him from moving forward or being on the field," Mozeliak said. "After we did our due diligence, we were comfortable and confident that this is not going to be something that is going to lag all season."
Masterson, who was scheduled to return to the Indians' rotation on Friday, recently made two Minor League rehab outings, including one last Friday in which he walked six and allowed five runs on four hits in 6 2/3 innings.
A year after going 14-10 with a 3.45 ERA and 1.20 WHIP, Masterson has a 5.51 ERA and 1.65 WHIP in 19 starts. He has struck out 93 and walked 56 in 98 innings. In nine of his starts, Masterson has allowed five or more earned runs. His velocity has also dipped.
"You also can't get too far away from what this guy did last year, not just statistically but look at his stuff a year ago," manager Mike Matheny said. "It was just filthy. You don't lose it that quick, especially when you're 29. Maybe there is a little bit of alterations. I know his previous team was doing the exact same thing, trying to get him back to the pitcher he was in 2013. There are very, very talented people over there. But if we have an opportunity here to put our best foot forward, we'll see how we can help him. Hopefully it brings out the best."
The Cardinals also believe that the ground-ball pitcher will benefit substantially from playing in front of an improved infield defense. Across the infield, the Indians have minus-39 defensive runs saved. In contrast, the Cardinals' infielders are at a plus-35.
Mozeliak acknowledged that the Cardinals were active in asking around about several other starting pitchers, including those considered to be the most coveted in-season trade targets. This deal came to fruition just hours before it was announced.
"Certainly, leaving [the ballpark] last night, I did not think we would end up with Masterson this morning," Mozeliak said. "There were some things going on, a lot of balls in the air. I talked with [Indians GM] Chris [Antonetti] early this morning and it really took traction then."
It was also a deal that did not cost the Cardinals anyone from their Major League roster. In trading Ramsey, the Cardinals were dealing from an area of surplus.
A first-round pick in the 2012 First-Year Player Draft, Ramsey has had a strong season in Double-A, hitting .300/.389/.527 in 67 games. He has been blocked for a promotion by the glut of young outfielders in the organization, however, which is why the Cardinals were willing to deal one of them as they looked to strengthen their rotation.
Ramsey, 24, represented the Cardinals in the SiriusXM All-Star Futures Game this month, but he has also missed time at three different junctures of the season due to minor injuries. Cleveland has asked Ramsey to report to its Triple-A affiliate.
"James Ramsey is a guy who we all really appreciated how he played the game and how he went about representing himself and this organization," Matheny said. "He had full buy-in to everything we believed around here, and he was having a great season. You hate to lose any young talent, but you also realize that [it's necessary] to pick up a pitcher the caliber of Masterson.