Leighton Baines scored the only goal of the game as Everton toiled against the Slovakian side at Goodison Park on Thursday in the first leg of their third-round qualifying tie.
Sigurdsson is reported to be valued at £50 million by Swansea City and the Iceland international did not go on the club's pre-season tour of the United States as he did "not feel in the right frame of mind to travel due to the current uncertainty over his future".
Koeman suggested Everton could make a fresh bid for Sigurdsson, who recorded 13 assists to help Swansea survive last season, but the Dutchman pointedly noted he is not in control of the club's transfer negotiations, with Jordan Pickford, Michael Keane and Davy Klaassen among the arrivals so far.
"It's up to the board," Koeman told ITV. "Everyone knows our interest in the player but I don't decide it in financial terms. It's up to the board."
Captain Baines settled the match with a deflected strike from just outside the penalty area and the left-back accepted Everton found it hard to break down a stubborn Ruzomberok in Wayne Rooney's second competitive debut for the club.
"We needed the goal. We wanted to score it earlier but we're thankful we've got it," Baines said to ITV.
"It will be a tough game over there. But if we keep a clean sheet we'll win the game.
"We're taking it seriously but it's a part of our pre-season too. That was the first 90 minutes for many of us."