Rodriguez missed time with elbow tendinitis, then had surgery in late June to remove loose bodies from the elbow. No timetable has been given on his return, but an earlier estimate had Rodriguez returning in September.
To make room for Niese, the team optioned pitcher Logan Verrett to Triple-A. Verrett produced a 2.96 ERA over 21 1/3 innings this season.
Storen has had a terrific season so far as the Nationals' closer. He is 29-for-31 in save chances and has a 1.73 ERA this season.
Still, the acquisition will give the Nationals a formidable back-end of the bullpen with a pair of stud closers at manager Matt Williams' disposal.
Mejia tested positive for Stanozolol and Boldenone. He had already served a previous 80-game suspension earlier this season. Mejia is 1-0 with a 0.00 ERA over 7 1/3 innings this season.
He returns with a 1-5 record and a 4.00 ERA through his first nine starts of the season.
In a corresponding move, the Brewers optioned right-hander David Goforth to Triple-A.
Papelbon is 17 for 17 in save opportunities this season with a 1.59 ERA. Papelbon will take over closer duties for the Nationals with Drew Storen being used a setup man.
The Nationals will send pitcher Nick Pivetta to the Phillies, reports the New York Post. Pivetta has been pitching at Double-A.
Adames has not appeared in the big leagues yet this season, but he owns a .296/.342/.416 slash line with 16 doubles and seven home runs in 86 games with Alburquerque.
Miller made eight relief appearances for the Rockies earlier this season, posting a 1.29 ERA over seven innings of work.
Laffey, an eight-year veteran, pitched 2 1/3 innings in a relief outing earlier this year. He is 2-3 with a 4.78 ERA in 58 1/3 innings with the Isotopes.
"I'm going to go down that path and see where it takes us," Stewart said.
Yahoo Sports reported earlier Tuesday the two sides were discussing a deal, and the Diamondbacks were doing their due diligence about the lefty reliever.
To make room for Beeler, the Cubs optioned Yoervis Medina to the minor leagues. Medina produced a 4.71 ERA over 21 innings of work.
Banuelos was scheduled to start on Wednesday.