Vincenzo Montella was left exasperated with referee Luca Banti after he failed to dismiss Rodrigo De Paul just two minutes before the Udinese winger netted the winner in a 2-1 defeat of AC Milan.
The Argentinian midfielder caught Mattia De Sciglio with a crude tackle in the 71st minute and only received a yellow card, with the Milan full-back forced off injured shortly after.
De Paul then found the bottom-left corner from 25 yards as Udinese caught Milan on the break, a goal which ultimately settled things in favour of the home side.
And Montella was adamant after the full-time whistle that De Paul should not have been on the pitch to score the winner.
"The De Paul situation added insult to injury," Montella told Mediaset Premium.
"As well as it being a red card, we ended up down to 10 men with an injured player and didn't have time to make the substitution. What can you say about that foul? It seemed a clear red.
"At the same time, we were naive; we gave the ball away and left ourselves open in that situation.
"We did something similar in Turin [a 2-1 loss at Juventus in the Coppa Italia] and you pay for that at this level. We hope it'll help these young players to grow and learn."
The result marked a third Serie A game without a win for Milan, who sit seventh in the table.