Ross Detwiler could be in line for a spot start next week. (John McDonnell/The Washington Post)

The Nationals know they need a starting pitcher Tuesday. They just don’t know who it will be, Manager Matt Williams said.

With Doug Fister set to make his debut Wednesday, barring a setback in his five-inning rehab start Friday, the Nationals’ rotation currently looks like this:

Wednesday: Jordan Zimmermann

Thursday: Off

Friday: Stephen Strasburg

Saturday: Tanner Roark

Sunday: Gio Gonzalez

Monday: Jordan Zimmermann

Tuesday: Undecided

Wednesday: Doug Fister

Thursday: Off

Friday: Stephen Strasburg

It’s notable that Strasburg will receive a full week between starts as Fister gets inserted into the rotation. More pressing, the Nationals need to come up with a starter Tuesday.

“We don’t know,” Williams said. “That’s the day we would potentially need somebody. We could push guys back, depending on how they feel. We could go on regular rest and make a call-up. We don’t have a definite plan yet. We’re working on it.”

The easiest choice would be Ross Detwiler, the left-hander who was part of the Nationals’ rotation last season and who is currently receiving little usage in consequential situations. But Williams does not want to commit to him in case the Nationals need Detwiler leading into Tuesday.

“You don’t want to not go after a game prior to save them for that game,” Williams said.

Williams also mentioned Craig Stammen as an option and said the Nationals could tap Class AAA Syracuse for their starter. The only viable candidate would be Blake Treinen, who would be on seven days rest for Tuesday.