The Boston Red Sox fired pitching coach Juan Nieves on Thursday, but have yet to decide on his replacement.

Nieves, 50, joined the Red Sox in November 2012. While he was part of the 2013 World Series champions, he is overseeing a pitching staff that is now the worst in the American League with a 4.86 ERA.

“We came to the conclusion change was needed … even if the replacement was not in place,” said Red Sox general manager Ben Cherington.

“If you look at the aggregate performance of our pitching staff … we needed a new voice.

“We believe in the guys on our team and that we will play better.”

Cherington said he has a “very small list of people in mind” to replace Nieves, saying there was one external and one internal candidate.

A couple of internal possibilities include Pawtucket pitching coach Bob Kipper, 50, the Portland Sea Dogs pitching coach the previous five years; and Bob Tewksbury, 54, one of Boston’s mental-skills coaches. Both have pitched in the majors.

In a separate move, the Red Sox are planning to designate reliever Edward Mujica for assignment, according to media reports out of Boston.

Mujica, who turns 31 on Saturday, is in the second year of a two-year, $9.5 million contract with Boston. In 11 games with the Red Sox this season, he has compiled a 4.61 ERA and a 1.32 WHIP.

Cherington said he expects to announce the move before Friday’s game in Toronto.

Righthander Matt Barnes, recently moved to the bullpen in Pawtucket, is a leading candidate to replace Mujica on the Boston roster.