Grilli, who left Monday night's game at Washington in the ninth inning with discomfort in his forearm, traveled to Pittsburgh Tuesday and will be examined by team doctors.
"The biggest thing we've got to do is get Jason back to Pittsburgh and be seen by our doctors and get a more thorough evaluation," Pittsburgh general manager Neal Huntington said. "The initial read was he's going to be down at least seven to 10 days so the DL becomes a no-brainer at that point, but the rest is speculative and anything else at this point would be purely that."
Huntington was asked if the initial read on the injury was encouraging or incomplete.
"Both," he said. "The initial read is encouraging but until we have the actual read it doesn't make a ton of sense for us to be out in front and speculate on what it is or how long he's going to be down."
The All-Star right hander is 0-1 with a 2.34 ERA and 30 saves.
Manager Clint Hurdle said Mark Melancon (2-1, 0.97, two saves) will assume the closer's role in Grilli's absence.
"I think it's going to be somewhat of a seamless transition. We can't replace Jason but we are going to move Mark to the closer spot," Hurdle said.
He said several relievers will work to fill Melancon's eighth-inning role.
"I do think we'll go the way the game flow is. We could have a different option every night," he said. "I don't think I'm going to settle into one guy right now."
The Pirates also plan to recall right hander Vic Black from Triple-A Indianapolis, where he's 1-2 with 15 saves and a 2.31 ERA.
In a separate move Tuesday, Pittsburgh activated second baseman Neil Walker, who had been on the disabled list since July 7 with right side soreness. Walker is hitting .244 with six homers and 26 RBIs this season.
To make room for Walker, Pittsburgh designated veteran infielder Brandon Inge for assignment. Inge appeared in 50 games, hitting .181 with a homer and seven RBIs.