Their poor league form in recent weeks has seen Hughes' men go from fighting for the European qualifying places in seventh spot to 11th in the league with 13 games remaining.
"Since the Liverpool game we have been poor, I'm afraid, for whatever reason. It's hard to know the reason why, but collectively we have underperformed," Coates told British media.
"We are not scoring enough goals and not creating enough chances, but I'm sure Mark and his team are well aware of that and are working hard to put it right."
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The Potters have failed to score in their last four league matches and have conceded nine goals in the process.
Stoke visit lowly Bournemouth on Saturday.
(Reporting by Shravanth Vijayakumar in Bengaluru; Editing by Ken Ferris)
