Manager Buddy Bell announced that Nunez will start on Tuesday night against the Red Sox, replacing injured John Thomson. He's been placed on the 15-day disabled list.
Musser, just optioned to Triple-A Omaha on Saturday, was recalled to replace Thomson on the roster for Monday night's game. But Bell said the plan is to have Musser leave again Tuesday when Nunez is activated.
"We thought we needed somebody else in the bullpen tonight," Bell said during Monday's early workout at Fenway.
Nunez, a right-hander, pitched in relief for the Royals in the previous two seasons. He appeared in a total of 48 games and had a 3-2 record and a 6.99 ERA.
In Spring Training, he suffered a broken left wrist and started the season on the disabled list. Upon his return, he was transformed into a starter and had a combined 2-1 record and 1.40 ERA for two Minor League clubs.
For Double-A Wichita, Nunez made five starts in six games and was 1-0 with a 0.87 ERA. For Omaha, he was 1-1 with a 2.00 ERA in four games (three starts).
"He's been throwing good. He comes well recommended," Bell said. "He's a competitive little son-of-a-gun."
Nunez came close to being an ex-Royal on June 22. He was traded to the Oakland A's for outfielder Milton Bradley, but the deal was voided because Bradley was injured.
A long wait: Sanders had missed 62 games while recovering from a torn left hamstring. He went on the disabled list May 3.
"It felt like a hundred years," he said, "but Mother Nature will let you know."
Before his injury, he had a .367 average in 16 games with two home runs and six RBIs.
The Royals are expected to test the trade market for Sanders with just 17 days left before the July 31 non-waiver trade deadline. Some clubs, including the Red Sox, could be interested in a veteran outfielder who could help down the stretch.
"That's what I'm hoping," Sanders said.
Sanders recognizes that, at 39, he has no place in the Royals' swing toward youth.
"Is there a purpose for me here, if you look at it?" he said.
Sanders said he had no clue about how the Royals might use him. He was on the bench when Monday night's game began. Bell said he'd try to play Sanders as much as possible.
When optioned, Gathright was batting .315 in 25 games with nine RBIs and five stolen bases. He was the swiftest player on the Royals' roster.
"We all hated to lose Joey. He did a great job for us while he was here," Bell said. "But, at the same time, we're glad to have Reggie back."
Bunts: Thomson, who made two starts for the Royals, isn't quite certain what's wrong with his back. He began feeling pain last Saturday, a day after throwing in practice at Cleveland. "It's a catch in the middle of my back," he said, explaining that the malady comes and goes. ... Former pitcher Curtis Leskanic, popular, witty and offbeat in his Royals years (2003-04), has joined the Red Sox as a pro scouting consultant. His last club was the 2004 World Series champion Red Sox. ... Left-hander Jorge De La Rosa's sore left thumb, which forced him out of Sunday's game, is better and the bruise has faded away. He's not expected to miss his start Saturday night at Detroit. ... To make room for Sanders on the 40-man roster, pitcher Sean Thompson was designated for assignment. ... After setting a club record with strikeouts in 22 straight games, Mark Teahen did not fan Sunday at Cleveland. "When I found out about the record, I decided to stop it right there," he said with a grin.
Royals rundown: Luke Hochevar, the No. 1 Draft pick of 2006, lost his first start for Omaha on Sunday. In five innings, he gave up five runs on five hits including three homers. He had seven strikeouts in a 13-2 loss to Iowa. ... Mike Stodolka (.291) had three hits as Wichita beat Arkansas, 5-3. ... Matthew Morizio's RBI single for Class A Burlington (Iowa) broke a ninth-inning tie in a 7-5 win over Great Lakes. ... David Lough had three hits including his first homer as Rookie Burlington (N.C.) beat Johnson City, 8-2.
Coming up: Nunez makes his first career start for the Royals at 6:05 p.m. CT on Tuesday at Fenway Park. He's matched against Red Sox knuckleballer Tim Wakefield (10-8, 4.47 ERA).