PHILADELPHIA -- The Nationals promoted right-hander Austin Voth from Double-A Harrisburg to start Tuesday's series opener in Baltimore. To make room on the roster, catcher Spencer Kieboom was optioned to Double-A.

Voth stepped in to the spot vacated by Max Scherzer, who was placed on the injured list Saturday with mid-back tightness. As for Wednesday's game against the Orioles, the Nationals could turn to Stephen Strasburg on regular rest -- thanks to Monday's off-day -- or call on someone else to make a spot start and save Strasburg for Friday's series opener against the division-leading Braves.

"As of right now, on Tuesday, Voth will start, and that's as far as we've gotten right now," manager Dave Martinez said prior to Sunday's finale against the Phillies.

Voth had an encouraging season debut for the Nats on June 23, holding the Braves to two runs over six innings, while striking out seven and walking none.

He had trouble duplicating that success in his next two outings, each of which lasted just 4 1/3 innings. He allowed seven runs in those starts for a 7.27 ERA, while striking out nine and walking five in 8 2/3 frames. Voth will be starting on regular rest Tuesday after tossing six scoreless innings for Harrisburg on Thursday.

"I like what Austin’s done when he’s up here," Martinez said. "And his last outing down there, he pitched really, really well. I like the fact that he goes out there, and he has presence. He has poise. He’s throwing the ball well, so we’re going to give him an opportunity to start."

Doolittle not worried about velocity

Closer Sean Doolittle admitted it was a bit surprising Friday to see his fastball sitting mostly at 91-92 mph. The same thing happened again on Saturday, though he worked a scoreless inning in both appearances.

"I’m not exactly sure why it’s down, but I know from past experience not to panic if I see the 91, 92," Doolittle said. "I feel pretty good. Everybody gets a little bit tired around this point in the season, but if I stay in my mechanics and I don’t try to overthrow, I still can get that life and deception on my fastball, and I can still navigate innings and get guys out."

Another encouraging sign for Doolittle was that he was able to ramp it later in the ninth inning Saturday, firing a 95 mph fastball on his second-to-last pitch of the night. That's above his season average of 93.5 mph, according to Statcast.

Still, eight of Doolittle's 20 pitches on Saturday came in at 92 mph or slower. That was one night after four of his 12 fastballs did not exceed 92 mph. Those are his only two outings this season in which he's thrown at least 10 fastballs and more than 30 percent were 92 mph or slower.

"[Friday] it started down too, but then at the end, he pumped it up a bit," said Martinez, who added that it would be "ideal" to avoid using Doolittle in Sunday's series finale. "As long as he’s throwing his fastball and he’s throwing it up, he’s effective. I know he’s been mixing a little more splitters and sliders, but when his fastball is up, he’s good."