Marco Silva believes Jordan Pickford should have been awarded a free-kick for the incident leading to Sheffield United's opening goal on Saturday.
The Everton goalkeeper missed an attempted punch clear only for a deflection off Yerry Mina to hand the newly-promoted visitors the lead.
Pickford was closely followed by United defender Jack O'Connell when the corner was delivered and Silva claimed Simon Hooper's officiating was uneven.
"I don't like to find excuses for that moment but in some moments inside the box our players they touched a little bit on them," the manager said.
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"Why if they do what they did on our goalkeeper in the six-yard box was it not a foul?
"For me it looks a free-kick for us but it is the referees decision that we have to respect but in the opposite box there was all these fouls in these type of moments and not with the goalkeeper."
Everton's defending from set-pieces was called into question last season but the Blues possessed one of the best defensive records in the Premier League before conceding nine goals in their last four games.
With issues raised once more, Silva called for hard-work on the training ground from his squad to repeat the trick of the previous campaign.
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"I can look into it," the 42-year-old added. "What we are talking about, set-pieces and all these things, we have sorted the situation and then last game comes the same conversation.
"If you look to the past we had sorted and we were strong for long periods of last season and the beginning of this season as well.
"We have to look for development to create again a conversation about the set-pieces. We have to keep working really strong and do things our best we can in that moment."
