The right-hander allowed two home runs in one inning of work Sunday in his first outing since landing on the 15-day disabled list with a plantar fascia strain in his left foot.
"He is doing fine," manager Davey Johnson said. "He will probably go an inning, or maybe a little bit more than an inning. We will see how he does and take it from there."
Johnson wouldn't speculate if Mattheus will return to the Nationals once activated, and stressed the club will take a wait-and-see approach with the 28-year-old.
"The main thing is not to rush him," Johnson said.