Ange Postecoglou said that Timo Werner's performance was "not acceptable" during Tottenham's 1-1 draw with Rangers in the Europa League on Thursday.
Werner, making just his eighth start in all competitions for Tottenham this season, was replaced at half-time by Dejan Kulusevski after a dismal first 45 minutes at Ibrox.
While the German had the joint-most touches in the opposition box (four) of any Spurs player at full-time, his overall play saw his side struggle in the final third.
Werner lost possession 16 times, with only Pedro Porro (20), who played the full 90 minutes, doing so on more occasions against Philippe Clement's side.
He also failed to complete any of his three dribbles and did not win any of his five duels, while also managing to complete just 18 of his 26 passes, an accuracy of just 69.2%.
Kulusevski, Werner's replacement, went on to score the equaliser for Spurs, cancelling out Hamza Igamane's opener two minutes after the break.
"When you've got 18-year-olds, it's not acceptable to me," Postecoglou said shortly after the draw at Ibrox. "I said that to Timo, he's a German international."
Werner, who is on loan from RB Leipzig, has struggled for form this term, managing just one goal and three assists in 19 appearances in all competitions.
Postecoglou is also without forwards Richarlison and Wilson Odobert, and he said despite his display, his lack of options means he may start on Sunday against Southampton.
"In the moment we're in right now - it's not like we've got many options, I need everyone to at least be going out there and trying to give the best of themselves.
"His performance in the first half wasn't acceptable.
"We need everybody, including him, to be contributing because we don't have the depth to leave people out if they perform poorly.”
Thursday's draw leaves Spurs outside the top eight in the Europa League table on goal difference with two games remaining against Hoffenheim and Elfsborg.
Postecoglou's side have also now won just one of their last eight games in all competitions (D3 L4), and are winless in their last five (D3 L2).