BOSTON -- The Red Sox placed pitcher Clay Buchholz on the 15-day disabled list Wednesday with a hyperextended left knee and recalled reliever Alex Wilson from Triple-A Pawtucket to take his place.

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.

Clay Buchholz issued eight walks and tweaked his knee Monday, eventually exiting after just three innings of Boston's 8-6 win. Dale Zanine/USA TODAY Sports

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."