The hard-throwing 21-year-old right-hander will start the season with Triple-A Reno.
''We saw a very talented young man,'' manager Kirk Gibson said in announcing the decision on Thursday. ''We put a lot on him. We asked him to try a lot of things. He tried them. Overall, we thought the right thing is to send him back to the minor leagues and finish off.''
Bradley was competing for the fifth spot in the Arizona rotation, which apparently will go to right-hander Randall Delgado.
''I'm not necessarily saying I saw it coming,'' Bradley said. ''I wanted to be here as long as I could. I think I did that. I understand being sent down. There are some things I need to work on and get better at. Go take care of business and get back up here.''
He was 2-1 with a 4.32 ERA in 8 1-3 innings this spring. He struck out 10 and walked eight.
''We want him when he comes up to stay here when he comes,'' Gibson said. ''He just has some things to simplify. He's close. He's somebody we look at who could really help us throughout the year. We think this is the way to go about it.''
Bradley was a combined 14-5 with a 1.84 ERA with Class A Visalia and Double-A Mobile last season.
This was the first big league camp for Bradley, the seventh overall pick in the 2011 draft, who threw in an intrasquad game on Thursday.
''Camp overall was a huge learning process. Learned a ton. Learned a ton about myself, what I need to do to compete and stay up here,'' Bradley said. ''Just be more consistent, that's the biggest thing. That's the thing I did my first two starts. I was very consistent in everything I did. The last two, I kinda got away from that. Lost control a little bit and couldn't really get it back.
''I threw today, threw good, threw as good as I did my first two. Just carry that over and carry that down to Triple-A.''
Gibson said the No. 1 thing Bradley has to work on is his command of his fastball, which has been clocked in the upper 90s.
''He gets in bad counts, then he leaves the ball up and it gets hammered,'' Gibson said. ''He has better stuff than that.''