Ronald Koeman has confessed that he is "worried" after seeing his Everton side succumb to a 3-0 loss in their Europa League opener against Atalanta BC on Thursday night.

The Toffees were second best throughout at the Stadio Atleti Azzurri d'Italia, conceding three times in the first half on what proved to be a disappointing return to continental football.

Everton have now failed to win any of their last five matches in all competitions since August 17, while also failing to find the net in the last three, leaving Koeman under no illusion that a vast improvement is required.

"I'm worried after what I saw yesterday," he told reporters. "I wasn't worried after Chelsea (a 2-0 defeat). The reaction should have started in the second half against Tottenham [last weekend]. But that and the first half against Atalanta is enough reason to be worried.

"We had a bad run for a time last season, and I have confidence we can get out of it. We have one point less from last season from the same games vs. Chelsea, Man City and Tottenham.

"But yesterday was a wake-up call. After this press conference I'll have a meeting with the key players. It's time for the experienced players to stand up."

Everton, currently 16th in the Premier League table, return to action on Sunday afternoon with a trip to face Manchester United at Old Trafford.