The Japanese youngster, initially earmarked for Borussia Dortmund's second team, produced a standout 45-minute cameo in Saturday's 3-1 pre-season friendly win over Rot-Weiss Oberhausen, scoring a goal and setting one up after coming on.
The BVB-focused Ruhr Nachrichten has already dubbed him the club's new "audience favourite", drawing comparisons with a young Shinji Kagawa, who arrived from Japan in 2010 as an unknown quantity before becoming a star.
Yamamoto has a long way to go before matching that trajectory, but head coach Niko Kovac was impressed by what he saw, insisting "the door upstairs" remains open for the teenager despite his primary assignment with the reserves.
Yamamoto's emergence offers a timely boost for a squad still missing the bulk of its World Cup contingent, who remain on holiday following the tournament.
With senior numbers thin on the ground for pre-season fixtures, the teenager's cameo has given Kovac an early selection headache to look forward to once his first-team players return, even if the coach was careful to note he had barely seen the youngster in training before his debut appearance.