Zirkzee was a 68th minute substitute for United, but couldn't find a winner for the visitors.
"It's something that worries us," said Amorim afterwards Sky Sports. "We were in the final stages. Then Joshua must understand that he must dive into the penalty area to complete the crosses.
"It makes me feel like we are doing things that don't make sense at certain times. And we have to work on that."
Former United midfielder Owen Hargreaves, in his role as pundit, said: "He was standing in front of me and he was very frustrated. I'm not going to lie about that.
"He was not happy with Dalot's positioning in the first half. And the positioning of Zirkzee in the second half also made him somewhat frustrated. Many of these players will no longer play in three, four or five weeks.''
- Get breaking football news faster with the new Tribal Football app! Download now on iOS and Android to stay ahead of all the latest transfers and football updates. Download now: App Store, Google Play