Roberto Martinez could not hide his anger at referee Mark Clattenburg's decision to award Stoke City a late penalty that sent Everton to a dramatic 4-3 defeat against Stoke City.
John Stones was adjudged to have brought down the Austria international deep into stoppage time on Monday, as the visitors came from behind to claim the points.
Arnautovic blasted home from 12 yards to give Stoke - who had led twice before going behind 3-2 with 19 minutes left - the victory, but Everton boss Martinez was unable to hide his frustration with Clattenburg's crucial call.
"There is real anger at the final decision; the penalty is never a penalty," he told the club's official website. "It's a situation where it cost us the draw. We would have been disappointed with that but to not even get the draw is tough to take.
"John touches the ball - slightly - but it is a touch of the ball. It is a very soft decision in that respect.
"We are disappointed to be losing a point so late in the game, especially when it was a similar action earlier in the game with Seamus Coleman and their left-back [Erik Pieters]."
Martinez added: "Overall we showed an incredible reaction to come back and get in front, but at 3-2 we should have managed the game a lot better," he added.
"It is the hardest thing to do, to get back on level terms, then get ahead.
"It was a real blow to lose the third goal but clearly we have to become stronger defensively. There are certain actions we have to defend a lot better and, as a team, we have to get more mature.
"We have to be wiser in protecting that lead."