Snell, 27, is 2-8 with a 5.36 ERA in 15 starts for the Pirates this season. He hasn't won in his past 12 starts. He had walked 44 and recorded 52 strikeouts in 80 2/3 innings.
Snell was scheduled to make his next start against Kansas City this weekend.
"I don't want to be a negative person," said Snell, frustrated with what he saw as negativity by the media and fan base directed toward him. "I need to be a positive person if I'm going to be here. I don't want to ruin this team. I just made a better decision for myself and for my career and better for my life. Why not do it now than wait until later when everything blows up?"
With Snell, who had one option remaining, back in the Minors for the first time since 2005, the Pirates have recalled starter Virgil Vasquez to assume a rotation spot. Vasquez will start on Friday night against Kansas City. Charlie Morton, who continues to be bothered by a nagging left hamstring injury, had his start pushed back to Sunday.
Snell will continue to earn his $3 million salary in the minors.
"It's me having to be myself and learning to control myself and not worrying about other things," Snell said. "If I have to stay down there for the rest of the year, it doesn't really matter. Whatever makes this team happy and is best for them. I wish all these guys the best."