Astacio tossed two innings and gave up two runs against the Orioles on Saturday before leaving the game to be examined by Dr. Bruce Thomas. The pain got worse when Astacio tried to throw his fastball. At one point during the game, it was clocked at 84 mph.
Astacio then went back to Washington to have an MRI taken by Dr. Ben Shaffer. The results are expected to be announced on Sunday.
"We are hoping it is just strain, and usually, with those types of injuries, you have to rest it for seven to 10 days and get into a throwing program to make sure [it's OK]," Dr. Thomas said.
Astacio was scheduled to pitch against the Mets on Thursday at Shea Stadium, but Ramon Ortiz will now make the start, with Tony Armas Jr., originally the No. 5 starter, moving up a notch and pitching against the Astros on Friday.
The Nationals signed Astacio to a one-year, $700,000 free-agent contract, plus incentives, on Feb. 27 because right-hander Brian Lawrence is out for the season with a torn labrum and rotator cuff in his right shoulder. At the time, Washington was not concerned with the fact that Astacio was a combined 6-10 with a 4.69 ERA with the Rangers and Padres last year.
The Nationals recognized that Astacio, 36, was one of the best pitchers down the stretch for the Padres, who won the National League West title in 2005. He went 4-2 with a 3.17 ERA for San Diego, including a 2-0 mark with a 1.89 ERA in the final month of the season.
But Astacio hasn't pitched well in a Nationals uniform during Spring Training. He has given up 18 runs in 13 innings.
Bergmann already has big-league experience, having pitched for the Nationals late last season. He was set to join the Zephyrs after Saturday's game, but general manager Jim Bowden and manager Frank Robinson gave him the good news about staying in the big leagues in a closed-door meeting.
"I was going somewhere to play baseball -- that's what it's all about," Bergmann said. "I would have been just fine going down to New Orleans. ... It's sad to say that I capitalized on a loss, but I get to go up for a little while. I don't know how long. I will do my best to stay up there."
The Nationals currently have four starters in the rotation, but they don't need a fifth one until April 9 against the Astros at Minute Maid Park. Bowden said either right-hander Ryan Drese or left-hander Billy Traber would make the start. They are expected to pitch in Minor League games on Monday and Tuesday, respectively. But according to a baseball source, Washington's think tank, which includes Bowden and Robinson, has already agreed that Drese will be the first option for the fifth spot.
"We haven't replaced Pedro," Bowden said. "We don't need to. So we'll go with the four starters we have and add the extra guy to the bullpen. We'll add the fifth starter when we need to."
Drese is currently on the disabled list. Last September, Drese had shoulder surgery for cartilage damage in his right shoulder. The Nationals had Drese slowly work his way back into shape during Spring Training. In fact, he didn't pitch in a game until Thursday. When he is at his best, Drese throws strikes and gets a lot of ground-ball outs.
Traber was a pleasant surprise this spring after signing a free-agent contract. He was 1-2 with a 2.25 ERA in four starts. But there is a split within the organization on what role he should play. While some believe he should be in the rotation some day, others think he is a reliever because he doesn't have enough pitches in his repertoire to be a quality starter.
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.