Leicester suffered a narrow 1-0 loss to Atletico Madrid in the first leg of their Champions League quarter-final.
The Foxes were unlucky to fall behind on 28 minutes as Antoine Griezmann scored from the penalty spot.
Replays showed Marc Albrighton actually made contact with Griezmann outside the box but the referee gave the spot-kick anyway.
Craig Shakespeare was not impressed with the penalty awarded to Atletico Madrid Dramatic images as Leicester City fans CLASH with police in Madrid Wed, April 12, 2017
Photos and video from Madrid show riot police confronting Leicester fans as blue smoke flares are set off and chants can be heard.
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It [the penalty] has ruined out game plan. It is tough to take when it is so clear
And Shakespeare wasn't impressed with the decision to award the penalty.
He said: "I think it is a disappointing moment, a key moment. The referee has to get that one right.
"You can't guess. It is a definite free-kick but outside the box. Refs have a hard job but you want the key decisions correct.
"I think 1-0, we would have taken that before the game. We came for an away goal but you saw how good a team Madrid are. The tie is still alive."
Antoine Griezmann was fouled by Marc Albrighton outside the box Manchester United fans react to Antoine Griezmann's performance against Leicester Wed, April 12, 2017
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Manchester United fans are pretty excited on Twitter after seeing Antoine Griezmann's performance for Atletico Madrid against Leicester City in their Champions League quarter-final
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Manchester United fans react to Antoine Griezmann's display against Leicester
Goalkeeper Kasper Schmeichel also slammed the decision.
He added: "It [the penalty] has ruined out game plan. It is tough to take when it is so clear.
"We should have had something from this but we have to accept it.
"They might have scored from what should have been a free-kick but it is easier to score from a penalty."
Shakespeare's side welcome Atletico to the King Power Stadium for the return leg on Tuesday (7.45pm).