United were leading until Andre Onana conceded a late penalty.
Ten Hag said later: "I am very frustrated, this should not happen.
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"We had put ourselves in a good position to grab the three points by playing some good football, but then we throw it all away in the dying minutes. It's so unnecessary.
"It is both about bad luck, and then the fact that we have not been lucky on the refereeing front in recent weeks. We have had so many penalties against us. The sentence today was fair enough, but others have not been."