Montero has been starting while Matt Wieters is on the disabled list. So third-stringer Pedro Severino moved into the lineup against the New York Mets on Saturday, while Jhonatan Solano's contract was selected from Triple-A Syracuse so he can serve as the backup catcher.
In another move, the Nationals recalled utility player Matt Reynolds from Syracuse, giving them an additional player on the bench.
Romero appeared in two games this season for the Nationals, allowing three runs over two innings for a 13.50 ERA. In 55 appearances for Washington since the beginning of 2017, he was 2-4 with two saves and a 3.90 ERA.