Newcastle's winless start to the Premier League season extended to 13 games after a 2-0 loss at Arsenal on Saturday.
Bukayo Saka and the player who replaced him as a substitute, Gabriel Martinelli, scored goals in the second half to consign last-place Newcastle to a seventh loss of the season.
Saka ran onto a deft pass from left back Nuno Tavares and shot low and across goalkeeper Martin Dubravka into the far corner to put Arsenal ahead in the 56th minute.
Saka appeared to hurt his left leg soon after his goal and, after briefly playing on, was replaced by Martinelli in the 64th. The 20-year-old Brazilian scored within two minutes.
