Cashner left Friday's series opener in Colorado after six pitches. He struck out leadoff hitter Charlie Blackmon, but Green and pitching coach Darren Balsley didn't like the way he looked on the mound, and Green took the ball from him after his first pitch to his second batter.
"I just threw a pitch and it shot a shock down my neck and it didn't feel right after that," Cashner said of the pain that came on at the end of his bullpen session. He wanted to try and pitch through it, but Green stopped him and Cashner didn't argue.
"You always want to take the ball," Cashner said. "It was pretty painful to pitch facing the first hitter."
There had been no signs of an issue before his warm-up.
"To my knowledge, he was ready to pitch, fine going into the bullpen, fine in the bullpen," Green said. "And with about seven or eight throws left in the bullpen, he started to feel it. He thought he could kind of muster up the courage to pitch through it. I found out about it right before he went out for the start, right when we exchanged lineup cards. I talked to him, he said, 'Let me give it a shot.' We looked at five pitches and then kind of made that determination."
Cashner was treated by the training staff Friday and Saturday, and he saw a Rockies orthopedic surgeon Friday night. The Padres expect to send him home to San Diego to see their medical staff while the team moves on to Atlanta.
"[An MRI] will give us an understanding of what's going on in there," Green said. "He obviously wasn't well enough to pitch yesterday. We shut him down for the time being and try to build him back up."
Had he felt better Saturday morning, the Padres might have avoided a DL trip, but there was no improvement a day later.
"It's pretty sore," Cashner said. "I didn't sleep very well last night, but we'll see how I respond in the next few days."
Cashner is the third of the original five starters on the Opening Day roster to leave the rotation over the course of the last month.
"When we started the season, if we talked about the strength of the ball club, we would have said it was the starting rotation, with Tyson Ross, James Shields, and Andrew Cashner," Green said. "You look up and here we are in June and none of them are in the rotation at present. We've managed to fill the gaps at this point in time, but it's been a tall order."
The Padres activated Erik Johnson Saturday to take Cashner's spot on the roster. Johnson was part of the trade with the White Sox for Shields and was previously scheduled to be activated Saturday to pitch in place of Shields.
Green said Luis Perdomo will probably move from the bullpen to take Cashner's spot in the rotation. Perdomo made a spot start in Shields' place immediately after the trade last weekend, and five days later he pitched 5 2/3 innings of relief after Cashner's six-pitch start Friday.
"Perdomo obviously threw the ball well yesterday," Green said. "He's stretched out after two consecutive starts for us, and we like the way he's progressing."