"We are currently working on a replacement," said general manager Ben Cherington. "If that person's not in place by tomorrow night in Toronto, then [manager] John [Farrell] and the rest of the staff will handle the duties."
Cherington said he has a short list of potential replacements, including at least one who is internal and one who is external.
The Red Sox entered Thursday with a 4.86 ERA, the highest of all Major League teams but the Colorado Rockies (5.38). Boston is 13-15 and in last place in the American League East.
"We've been in the process of examining our overall pitching performance, and as everyone knows, there's a lot that goes into pitching performance, starting with the pitchers themselves," said Cherington.
"There are a lot of different factors that come into play when looking into why things are happening well or not well, and given the performance we've had to date, we've been looking at all of those things. And while we're confident that we have many of the pieces in place to improve ... John and I simply got to the point where we felt that, in order to continue to push forward and make improvements, we needed to make a change and have a different voice in that particular position."
Nieves, 50, drew rave reviews in 2013, his first year as the Red Sox's pitching coach, while helping lead the team to a World Series title. That year, Boston set a franchise record with 1,294 strikeouts.
For whatever reason, the fit no longer seemed to be as good as it was then.
"What it comes down to is focusing on the rotation, because the rotation pitches the bulk of the innings, quality required, stability needed," said Farrell. "Whether or not that same connection was made with the current group to bring about the consistency of performance ... as Ben stated, there's a number of things that go into this. But the ability to maintain the consistency or affect change when needed or [make] adjustments that are required that's typical with any pitcher, that wasn't the same as we saw ... two years ago."
Prior to working with the Red Sox, Nieves had spent 14 years in the Chicago White Sox organization, including five years as the parent club's bullpen coach. Nieves, a former big league pitcher with the Brewers, also coached for five seasons in the New York Yankees organization.
"First of all, I want to thank Juan for everything he did for the Red Sox," said Cherington. "He's a terrific guy, a quality coach and he was obviously a part of the huge success two years ago."
Lacking a bona-fide ace, the Red Sox were hoping they could get steady contributions from a rotation of Clay Buchholz, Rick Porcello, Justin Masterson, Wade Miley and Joe Kelly.
Porcello (4.38 ERA) is the only Sox starter with an ERA under 5.00. In addition, the righty has the club's only two wins since April 29.
"We believe in the guys that are on this team and we believe that we're going to start to play better, and that will come brick by brick," said Cherington. "I think we've seen, even in the last few days, a lot of good things from the pitching staff. Look, over the course of a season, adjustments are made, so I'm sure over the course of the rest of our season, there will be a time when some adjustments may be made, but right now, we're behind the guys we have.
"We believe there's a reason to feel good about what they can do going forward. We fully expect those five guys will continue to take their turn and try to build on what they've done."