The Premier League side were defending a 2-0 lead from the playoff first leg at Goodison Park, which had been marred by crowd trouble from the travelling supporters.

Wayne Rooney was back in action a day after announcing his retirement from international football with England, but it was the home side who threatened early on.

The Stadion Poljud was rocking just before half-time when Josip Radosevic reduced the deficit with a 30-yard effort.

The home supporters, though, were left in stunned silence less than a minute after the restart when midfielder Sigurdsson -- who completed his £45 million move from Swansea last week and came on as a substitute against Manchester City -- looped the ball in with an angled drive from just inside the Hajduk half.

Goalkeeper Jordan Pickford, another big-money summer signing after his £25m switch from Sunderland, then saved a penalty from Ahmed Said as Everton closed out a 3-1 aggregate win to take their place in Friday's group stage draw.

"I saw Gylfi's goal, but I don't think everyone on the bench did. It was incredible. I don't have any other word for it,'' Koeman said on Everton's club website.

"You need to be really clever to see that possibility and, after two or three weeks of the season, it could be the goal of the season.

"It was really important to be level so quickly in the second half.''

Gylfi Sigurdsson scored one of the goals of the young season on Thursday. Tony McArdle/Everton FC via Getty Images

Koeman added: "We dropped a little bit in the first half to be compact but I thought we did well from the start. The goal just before half-time made it difficult, but before that neither side created many chances.

"The penalty save was key as well because there were still 30 minutes to go at that time.

"Jordan is showing that we made the right decision to sign him. He is still a young goalkeeper and I have been really impressed by what he's done from day one here.

"It is an important position in the team. There's a big future for Everton and a big future for him.''

Koeman had made four changes for the European tie, handing starts to the likes of defender Cuco Martina, Muhamed Besic and Ademola Lookman in midfield.

"It was a big test for our younger players in the atmosphere. They will learn from it,'' the Dutch coach said.

"You see the older players, like Rooney, [Leighton] Baines, [Ashley] Williams, [Morgan] Schneiderlin and Sigurdsson, knowing what to do at the right times which makes it easier.''

Everton travel to Chelsea on Sunday, and Koeman revealed Senegal midfielder Idrissa Gueye had been rested as a precaution.

"It was too risky to start Idrissa Gueye,'' the Everton boss said. "He had a scan on his hamstring and everything was fine, but we didn't want to take the risk with Morgan Schneiderlin out of the game on Sunday [through suspension].''