"We have some pretty specific things that we're making sure that we consider during the game," Callaway told reporters Tuesday. "It's necessary ... to talk about guidelines and have a good sense of what we want to do or how we want to push them, even going into the week and not just going day by day."
Gsellman (6-2) has pitched 62 innings this season, which ranks fifth in the majors among all relievers, and has a 4.02 ERA. Lugo (2-1) has pitched 55 innings, which is tied for 16th among relievers, and has a 2.44 ERA.
Callaway said Lugo might rejoin the starting rotation this season and is ticketed as a starter when spring training begins in 2019.
"The main part is, we have to make sure that these guys are rested fully before they go out there and relieve or be rested fully before they go out there and start," Callaway said. "So, I wouldn't rule that out. September can be an interesting month, and if we have the opportunity or the need, he can definitely slip back in the rotation."