The decision came on an off-day, with the Red Sox four games below .500 at 59-63 and carrying the eighth-worst ERA in baseball (4.30).
"This was a performance-based decision," general manager Ben Cherington said on a conference call. "As I said yesterday and as I think Bobby has said, I think there's been a real good effort on the part of the staff to work together and iron out any communication issues that may have existed previously. We simply felt like we needed to make a change to put our pitchers in the best position to do what they needed to do the next six weeks.
"We feel like the next six weeks are important, no matter what our record ends up, and there are things we need to accomplish the next six weeks to create a foundation going into the offseason. We felt like this change was needed to give ourselves the best chance to do that."
Cherington said he did not envision making further changes to the coaching staff, which saw an overhaul just this past winter.
McClure, 60, was hired in November as a special instructor before Boston hired Valentine. McClure was named the pitching coach in December.
Controversy has surrounded Valentine and the team the entire season, and McClure and Valentine have at times offered differing opinions on the staff. Rotation leaders Josh Beckett and Jon Lester have both underperformed, and Daniel Bard's conversion from a reliever to a starter was a failure.
"Nothing in this year has anything to do with him, so it's, I guess, kind of the nature of the beast," Lester said. "I just talked to him and he seems to be in a good place, so that's all that really matters that he's good with it and moving on.
"You don't ever want the responsibility to be on those guys, it's our fault, pitching staff all the way down. You don't ever want the blame to be on those guys, they're just trying to make a living just like everybody else. It's a tough move, but the good part about is being able to talk to him and know that he was OK with everything and moving forward."
McClure, who was let go as the Royals' pitching coach after 2011, will not continue with the organization.
"We've relieved him from his duties, and he's no longer going to be working for the Red Sox," Cherington said. "We have great respect for Bob. He's a quality guy, a good coach. It just didn't work out the way we'd hoped. Whenever it doesn't work out, we have to look at ourselves first and ask what, if anything, we could have done differently to make it work better, so we'll do that, but it just wasn't working out. We felt like we needed to make a change. We felt like the right thing to do was to give everyone a fresh start, and Bob will get a fresh start and I fully expect him to get a good opportunity somewhere else."
Niemann spent the start of the season in the Red Sox's dugout along with McClure, but Major League rules on the number of in-game coaches forced Niemann out, until now.
Niemann spent 24 years with the Mets in a variety of roles -- from Major League bullpen coach to pitching coordinator -- and pitched for parts of eight seasons in the bigs from 1979-87, going 7-8 with a 4.64 ERA in 122 games and 200 innings.
"Randy's got a lot of experience, too," Cherington said. "He knows our guys well. He's been involved with the pitching staff pretty intimately since the beginning of Spring Training. There won't be any learning curve, that's for sure. He's done most jobs in the game. He was obviously a Major League pitcher himself and has had a long coaching career and has a lot of experience, and we felt like he can be part of the solution to making sure that we get a lot of good work done with our pitchers the rest of the season."
After John Farrell left the Sox to manage the Blue Jays following the 2010 season, Curt Young was hired away from the A's to be Boston's pitching coach. Young lasted one tumultuous season and is again Oakland's pitching coach this season.