Instead, Yost sold the move as similar to the one that sent outfielder Gabe Gross to Nashville in July to get consistent at-bats. Before Monday, when Villanueva pitched three innings, allowing no runs on no hits with one strikeout, he had a 10.80 ERA over his last 18 games and had allowed at least one run in five straight.
Yost said Villanueva's outings had become "forced."
"I knew something was going on. I had a feeling," Villanueva said. "Obviously, I'm disappointed, but what am I going to do? It is a little odd, but they are the bosses. I am going to do whatever they need me to do. I have no other choice.
"I'll go down. If you want me to start, I'll start. If you want me to relieve, I'll relieve. If you want me to hit, I'll hit."
Monday's move had been in the works for some time, but after burning through his bullpen over the weekend and with McClung unavailable before Tuesday, Yost needed Villanueva's fresh arm for Monday's game in Arizona.
It was getting difficult for Villanueva to work out his kinks in the middle of a pennant race, Yost said.
"These one-inning stints, it's hard to get any kind of command down," Yost said. "He's at the Major League level, you're battling for first place. He's just got a lot of pressure on him every time he steps out there. I wanted to get him in a little less-pressurized situation to refine his mechanics a little bit."
For the season, Villanueva is 6-3 with a 4.68 ERA. He lowered his ERA 18 points with Monday's performance.
"Tonight was, of course, the best outing he has had in a while," Yost said. "We need him. He's a very, very valuable part of our 'pen all summer long and he's been in a little bit of a struggle. He needs to sharpen up a little bit and come back in that spot where he can handle the sixth and seventh inning on a consistent basis."
McClung, acquired last month for reliever Grant Balfour, was 2-0 with a 1.42 ERA in five games, three starts, with Nashville. He is 17-24 in 85 career Major League outings.