The defeat heaps pressure on the coach.
“We have to analyse this game. First of all I am very sorry for the fans who came to the stadium, because we didn't win," De Boer told Sky Sport Italia.
“We lost the ball many times too easily and were very sloppy. Eder hit the woodwork and in the second half we had total control, but on a free kick (Jeison) Murillo was out at that moment and they blocked us to create space for the goal.
“Much like at Pescara, we lost our heads a bit after going behind. We started playing well in the last 20 minutes and were confident we could score a goal.
“I don't understand how we can lose our heads like that when we are just 1-0 down and there's time to recover. We should continue working and not lose control like that.
“I am angry because I'm convinced we could've scored the first goal and had the chances to do it. That's the way it went, I am confident we can improve."