On Tuesday, King entered the game in the eighth with the Nationals down, 4-0, and the Braves hit him hard. He gave up four runs on two hits and walked three batters in two-thirds of an inning. After throwing a couple of pitches to Brian McCann, King couldn't go any further because of the shoulder problems.
An MRI on Thursday revealed the tendinitis and King is expected to be out two weeks. The last time King was on the disabled list was in 2002, when he had elbow inflammation. He currently has a one-year contract with the Nationals for $850,000.
King has been a workhorse the last six years, averaging 78 games per season, and was relieved to know he didn't have a much more serious injury after pitching in so many games.
"You are thinking the worst, but you try to be optimistic," King said. "I finally get the MRI and I find out that it's tendinitis. Hopefully it could get knocked out the next four or five days, and then I would start playing catch and hope to be back on the mound by [the end of April].
"The shoulder felt better than it did yesterday, so that's a good sign. I will get on medication and start working on my range of motion."
By King going on the disabled list, it means that Micah Bowie will be the only left-hander out of the bullpen. Manager Manny Acta said that Bowie will now be a specialist instead of pitching one or two innings.
"Being the only guy there, we have to pick and choose the best spots to use him," Acta said.
Rivera, 29, was a quality reliever for Washington in 2006, posting a 3.43 ERA in 54 appearances, but he was one of the last players cut in Spring Training this year. Rivera pitched in only one game for Columbus, allowing one run in two-thirds of an inning on Sunday at Indianapolis.
Rivera is expected to be in uniform in time for Thursday's game against the Braves.
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.