Detwiler has been on the disabled list since July 4 with a herniated disc in his lower back.
Detwiler is currently in the middle of a 30-day rest period. The Nationals then hope to put Detwiler on a throwing program that will rebuild his arm strength.
Johnson remains hopeful that Detwiler will return before the end of the season.
"We'll try to bring him back before the year is over to make sure he's healthy, to make sure nothing further has to be done with him," Johnson told reporters before Saturday's game against the Brewers.
Detwiler is 2-7 with a 4.04 ERA this season.
Update: Jayson Werth's hot streak continued in the second inning with a leadoff single. Ian Desmond followed Werth's single with a one-out walk.
Werth and Desmond then pulled off a double steal against Brewers right-hander Donovan Hand. Following the double steal, Anthony Rendon hit a sacrifice fly to drive in Werth and give the Nats a 1-0 lead.
Prior to tonight's game, Werth was hitting .500 over the last seven games. He is now hitting .312 for the season.
Update II: Wilson Ramos' solo homer increased the Nats' lead to 2-0 in the fifth inning. Through four innings, Dan Haren has pitched four innings, throwing 71 pitches. He has allowed just two hits while striking out five.
Update III: Adam LaRoche hit a solo blast in the sixth inning to increase the Nationals' lead to 3-0. The homer was No. 15 on the year for LaRoche.
Werth left the game after the sixth inning with an apparent injury. MASN cameras showed Werth running slower than his normal pace as he was running the bases. Roger Bernadina has replaced Werth in right field.
Update IV: Haren is done after seven innings. He allowed four hits, walked two and struck out six. Since coming off the disabled list on July 8, Haren's ERA is 2.40.
Tyler Clippard is on to pitch the eighth inning.
Update V: Rafeal Soriano earned save No. 28 of the season after retiring the Brewers in order in the ninth inning. With the win, Haren improves to 6-11 on the season.
The Nats have won their second series since the All-Star break.