The 28-year-old centre-back appeared to elbow Fulham's Philippe Senderos during the club's 1-0 defeat at Craven Cottage on Saturday, a game in which Steven N'Zonzi was also handed a yellow card for an off-the-ball incident with Bryan Ruiz.
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"I don't think Robert will be pleased and it affects the team," the Stoke boss told reporters, "but everyone makes mistakes and he will be big enough and strong enough to hold his hand up and say that what he did was wrong.
"Robert has been fantastic for this football club - a great player and one of the signings we will look back on and say: 'What a wonderful signing'.
"But I think he has let himself down more than anything else."
The Potters currently sit in 10th place in the Premier League table and face fellow mid-table outfit West Ham at the Britannia Stadium on Saturday.