Buchholz has been struggling this season. The Sox say he hyperextended the knee during his last start, Monday against the Atlanta Braves, although he stayed in the game after waving off the Sox trainer.
The decision to place Buchholz on the DL came after he threw a bullpen session before Wednesday's game.
"When he got into the bullpen, probably the 18- or 20-pitch mark, there was some feeling of some instability," manager John Farrell said after Wednesday's 4-0 win over Atlanta. "Can't say that it's structural, but when he tried to get increased intensity and get some finish to pitches, there was that sensation that was being felt, and we shut him down at that time. As I mentioned before he went out to the bullpen, we weren't going to put him at risk if he felt anything coming out of that situation the other day. That's where he felt it; it was just with that instability trying to get through some pitches."
Prior to his session, Buchholz met with Farrell and pitching coach Juan Nieves in the manager's office. Afterward, Farrell met with former team psychologist Bob Tewksbury, who currently works for the MLB Players Association in the same capacity.
"Not at his peak," Farrell said of the pitcher's confidence. "And those are some of the things we continually work with him on, both in terms of the approach we take with other starters here, dealing with the mental side of the game."
In his past four starts, Buchholz has allowed 19 earned runs, 33 hits and 15 walks in 18 innings,.
"I felt it a little bit in the knee today in the bullpen," Buchholz said Wednesday night. "I think this will be more of a time, while I'm going through this, use the time to the best of my ability, as far as getting command back in all of the pitches that I throw. That being said, DL's the DL, and it's 15 days for me to get back to where I was, and I'm confident that's where I'm headed."
Throughout his career, Buchholz has dealt with injuries and mental struggles.
"That's where our history with him and being around him every day, you get a closer read on that," Farrell said. "It can't be done just through conversation. There's got to be some candidness and confronting some of the issues that might be there, and that's what we're working through right now."
Added Farrell, "Bottom line, it's in-game adjustments and execution, is what it boils down to."
Sending Buchholz to the minors is not in the plans for the veteran pitcher. He has been down that road a few times in the past and believes a stint in the minors will not fix what he has been dealing with.
"I don't know how much I'd gain pitching at the minor league level, just for the simple fact I've been there, done that, and I'm confident at Triple-A and Double-A, so it's a matter of getting big league hitters out," he said.
Instead, Buchholz said he'll spend the next few days watching video and trying to refine his delivery. He thinks he'll be back on the mound throwing bullpen sessions in the next four or five days.
Buchholz appeared confident that he'll be able to fix what's wrong.
"I know it's not an injury to my arm like it was last year, so first and foremost is health of that area of my body and the ball's coming out of my hand fine; it's just a matter of getting that little fire that I had last year, as far as throwing pitches in the zone, pitching to contact instead of pitching to swings and misses," he said. "It's hard to do that whenever you miss with your pitches, because when you miss, it gets hit hard. I've got to get back to the basics and build from top to bottom with it."
Earlier in the day, Buchholz spent time with Tewksbury. The two have known each other since Buchholz was drafted in the first round (42nd overall) in the 2005 draft. They've had many conversations in the past, and Wednesday's was just another one of those sessions.
"He's a guy who's been through a lot, and anybody that can pitch as long as he did in the big leagues [13 years] at 85 miles per hour has to have a good mental side of the game, so he's a good guy to pick at and asking him about the ups and downs he went through and how he prepared and how he got through it," Buchholz said. "Obviously he did a pretty good job doing that."