Fullkrug, who signed for Borussia Dortmund from Werder Bremen last Thursday, has sustained a "slight tendon strain" and is considered a major doubt for the upcoming friendly fixtures against Japan and France.
Germany have confirmed that the 30-year-old is set to travel with the rest of the first-team squad to Wolfsburg's Volkswagen Arena ahead of Friday's clash with Japan, but the striker may not be available for selection.
In the likely absence of Fullkrug, head coach Hansi Flick has opted to recall 33-year-old Muller to the senior set-up for the first time in nine months.
Muller has earned 121 international caps for Germany since his debut in March 2010, scoring 44 goals and registering 40 assists in all competitions.
The versatile forward has not represented his country since the 2022 World Cup in Qatar when Die Mannschaft suffered an embarrassing group-stage exit for the second successive tournament.
Muller, who has provided one assist in three appearances for Bayern Munich so far this season, is one of five Bavarian players to have been called up to the Germany squad along with Joshua Kimmich, Jamal Musiala, Leroy Sane and Serge Gnabry.
Flick's squad also includes two uncapped players in the form of Brighton & Hove Albion midfielder Pascal Gross, who has received his first-ever call-up at the age of 32, and 33-year-old Hoffenheim goalkeeper Oliver Baumann.
After facing Japan on Friday, Germany are scheduled to take on World Cup finalists France in a friendly clash at Westfalenstadion next Tuesday.