To fill his spot on the 25-man roster, right-hander Michael Schwimer has been recalled from triple-A Lehigh Valley.
"It's unfortunate," Amaro said. "We've been losing a lot of guys. But we'll adjust. We have to."
The injury occurred in Friday's seventh inning. Facing Omar Infante to lead off the inning, Contreras threw a fastball on a 2-2 count and then clutched his right arm and fell to the ground. He exited after throwing just five pitches.
The Phillies had one of their physicians examine Contreras after the injury and planned to have team physician Michael Ciccotti look at him on Saturday. Contreras had right elbow surgery in the offseason, but he told Amaro this injury does not feel as bad as the previous one.
"He said he felt great in the bullpen and had no issues, but then on that one particular pitch is where he really felt it, obviously," Amaro said. "We can't really speculate what it is right now, but we're likely to put him on the DL."
"He grabbed his elbow. That's not a good sign," manager Charlie Manuel said. "I think he got hurt pretty bad."
In 17 appearances this season, Contreras was 1-0 with a 5.27 ERA. The 40-year-old had pitched well of late, though. He was 1-0 with a 1.86 ERA in his previous 11 appearances.
If the veteran reliever does indeed go to the disabled list, the Phillies will likely call up a right-handed reliever to take his place as they already have four left-handers in the bullpen.
"We need some bullpen pieces," Manuel said. "We needed some when the game started [Friday night], and now we need more."
Three right-handed pitchers in Philadelphia's Minor League system are on the 40-man roster: Michael Schwimer and Phillippe Aumont with Triple-A Lehigh Valley and J.C. Ramirez with Double-A Reading. If the Phillies decide to promote a different pitcher, they would have to remove a player from their roster or move a player to the 60-day disabled list. Amaro did not rule out promoting a starting pitcher from the Minors to pitch in the bullpen.
"We'll have a decision for you [on Saturday]," he said.
Jake Kaplan is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.