Tyler Austin of the Yankees was suspended five games, but after appeal, MLB on Thursday trimmed that ban to four games. He will begin serving it Friday after playing in the Yankees' 4-3 win against the Twins on Thursday.
Kelly was suspended April 18 for his role in the brawl.
"It's disappointing,'' Kelly said. "Suspensions are always not fun. Went in there and went through the appeal process. Obviously it didn't work out too well.''
Both he and Austin threw punches after Austin rushed the mound at Fenway Park when he was hit by a pitch from the right-handed Kelly.
Following the decision, Kelly posted a message on Twitter that read:
At least take me to dinner first...— Joe Kelly Jr. (@JosephKellyJr) April 26, 2018
Despite the outcome, Kelly said he didn't regret appealing.
"I always like sticking up for myself,'' he said. "It kind of helps me sleep at night going in fighting for what I believe in.''
New York has won six straight, and Austin is batting .350 with three homers and nine RBIs in his past five games, raising his average to .290.
"I obviously don't want to be suspended at any point in time, but it's happened, we've got to deal with it and move forward,'' Austin said after Thursday's game. "I'm happy they knocked one game down. I'm thankful for that.''
Red Sox manager Alex Cora said he had expected a reduction in Kelly's suspension.
"If I was a gambling man, I would have bet a dollar that they would have at least reduced it to five, but it didn't happen,'' Cora said.
Kelly has a 3.09 ERA in 11 games this season. After allowing four runs in one-third of an inning in an opening-day loss to Tampa Bay, the right-hander has not allowed a run in 11 1/3 innings. Boston will have to play with 24 active players during the suspension.
"It's a tough time,'' Kelly said. "We're playing good baseball and, especially for myself, I felt like I was getting into a little bit of a rhythm.''
Kelly said he expects to throw a bullpen session to stay sharp during his suspension. He is eligible to return Wednesday against Kansas City.
Information from The Associated Press was used in this report.