Fister twisted his ankle in the sixth inning as he maneuvered to avoid a hit up the middle by Carlos Gonzalez. He exited the game later in the inning with two outs and the bases loaded.
"I tweaked it a little bit, but I'll be fine," Fister said. "It will probably be sore for a day or two."
Frandsen twisted his ankle during a pinch-hitting appearance in the ninth inning. He caught a spike in the dirt, then rolled his left ankle while making contact for a groundout.
Nationals manager Matt Williams said there was some initial concern about Frandsen injuring his left ankle because he suffered a ruptured Achilles tendon in the same region earlier in his career.
"That's the same ankle," Williams said. "So any time there is an issue there, it scares him, scares us. If you have another issue with that Achilles, it's not good. But he showed up today feeling good. It's sore, but he's OK."
Williams said Frandsen was available off the bench Tuesday.
The Nationals may have avoided their biggest injury scare in the seventh inning Monday. Ryan Zimmerman, headed home from second after a Wilson Ramos single, tumbled safely across the plate as Rockies catcher Wilin Rosario failed to corral the throw from Gonzalez.
As Zimmerman rolled behind the plate, he knocked over Ian Desmond, who was waiting on deck. After several moments kneeling in the dirt behind the plate, Zimmerman walked unharmed into the dugout.