Melvin also isn't afraid to use right-handers Ryan Cook and Dan Otero in save situations, should Doolittle or Gregerson's services be necessary beforehand.
"I think right now we're fine with this, but there's the potential for anything," said Melvin. "I know, at times, it's a little easier for guys in the bullpen to know exactly when they're going to pitch, but that's not where we are right now.
"Even with how I look at things sometimes, I'm mixing and matching a little more to try to get to who the closer is on that particular day, and it's a little different for some of those guys, but we feel like we have the guys who are able to do that."
Johnson is included in that mix, having bounced back from a rough beginning with five consecutive scoreless innings spanning three appearances.
"Johnson's really been the wild card for us, and fortunately for him he's been great," said Melvin. "He's pitching a lot better right now and showing that he can be used for more than one inning, too."
Jane Lee is a reporter for MLB.com. Read her blog, Major Lee-ague, and follow her on Twitter @JaneMLB. This story was not subject to the approval of Major League Baseball or its clubs.