Later Sunday, Gonzalez had an MRI that revealed no structural damage and confirmed the inflammation in the shoulder, according to manager Matt Williams.
"I spoke to him late [Saturday] night. He was bummed out a little bit, depressed," general manager Mike Rizzo said about Gonzalez. "That's his personality. He wants to help the ballclub. He is a great team guy. He is a vital part of this club. He has never been on the DL before. It's something that is new to him."
Gonzalez has been dealing with shoulder problems dating back to his April 23 start against the Angels, but he said it was nothing to be concerned about. However, he had one of his roughest starts Saturday in a 5-2 loss against the Mets, allowing five runs in three innings. After the game, he declined to discuss his shoulder problems, saying he had to gut it out on the mound.
"It's one of those things you've just got to grind through," Gonzalez said. "I'm just trying to find out what it is, and hopefully something positive comes out of it."
Rizzo thought Gonzalez looked fine during the start. The GM noticed that Gonzalez's velocity and spin rate on his pitches were normal, but his command wasn't right.
After the game, Gonzalez passed the strength test, but felt soreness in the shoulder. Combining that with the fact that he had soreness last month, the team decided to get the MRI to see what's going on inside the shoulder.
"He will be shut down for four or five days. No throwing, no nothing and then we'll start him back on the program throwing. We'll get him a bullpen when we can," Williams said. "… You look at him and say, it's bothering him. We have to send him for a test. The test came back great. So we'll build him back up. Love to have him back when he is ready."
The reason Gonzalez was put on the DL before the MRI was taken was that he had a dye put in the shoulder, meaning he can't throw for five days.
"We figured that he misses two starts. We'll find out what's going on in there and we'll take it from there," Rizzo said.
For now, the Nationals have recalled Mattheus, who will be in the bullpen. Mattheus returns for a second stint this season. In two appearances April 29 and May 5, Mattheus pitched three scoreless innings.
The 30-year-old Mattheus is 7-7 with 32 holds and three blown saves in 140 career appearances, spanning portions of four seasons (2011-14) with the Nationals.
In 13 appearances with Triple-A Syracuse this season, Mattheus has two saves and a 5.93 ERA. He has walked four and struck out 11.
"I [have to do] the same things I always do: Attack the strike zone," Mattheus said. "When I've had success at this level, I come in and get outs early in counts, use my strengths. When I get in trouble, I try to do things I'm not very good at. When I try to strike guys out, I try to use my offspeed pitches in the wrong situations. So when I come in and use the two-seam fastball, [I] attack hitters and get ahead, things like that. I think it will work out. Hopefully, I can stay here and contribute."
Gonzalez's turn in the rotation isn't until Thursday against the Pirates. Rizzo declined to say who would replace Gonzalez, but the Nationals could recall right-hander Blake Treinen to join the rotation.
Treinen has performed well in his two stints with Washington as a starter and reliever. He is 0-1 with a 0.77 ERA in four games. The last time he pitched for the Nationals was May 6 in an 8-3 loss to the Dodgers. In that game, Treinen pitched five innings and allowed three unearned runs.
Treinen is 2-0 with a 1.96 ERA in six starts for Syracuse.
Bill Ladson is a reporter for MLB.com and writes an MLBlog, All Nats All the time. He also could be found on Twitter @WashingNats. This story was not subject to the approval of Major League Baseball or its clubs.