The 27-year-old was placed on leave for a week last Thursday under the league’s domestic violence policy.
Germán is being investigated by the league for an alleged incident involving his girlfriend at his home last week and the extension allows the investigation to continue.
The alleged incident was reported directly to the league and there is no police report, an MLB source reportedly told the New York Daily News.
After Germán's initial suspension, the Yankees privately said they didn’t expect him to play in the playoffs, according to the Daily News.
Once the investigation is completed, Germán could be back next season.
He was 18-4 with a 4.03 ERA in 24 starts and three relief appearances over 143 innings this season.
Germán' will be paid his $577,500 salary during the administrative leave, but it could later be turned into an unpaid suspension.
He has been a vital pitcher for the Yankees in a season in which many of their star players have seen time on the injury list.
The Associated Press and Fox News' Ryan Gaydos contributed to this report.