Harford had sprung a surprise before kick-off as he handed German his first ever start for Rangers, but the 18-year-old did not disappoint, worrying Doncaster throughout with his pace and physicality.
German was rewarded after 25 minutes when he showed a poacher's nose for goal by flicking home Rangers first goal in their 2-1 success at Loftus Road.
"It was hard work but I thought overall we performed very well," caretaker boss Harford said after the match.
"I thought German was the best player on the pitch.
"He's technically very good and if he continues to improve he's got a good future."