Iceland will play Nigeria in Volgograd on Friday.
"It's very unlikely that he [Gudmundsson] will be fit. He's better than he has been, he's getting better every day but I'm not going to hide that it's very unlikely that he will play. We have been rotating the team a lot during the preparations and many people have been given a chance and they've done it well so we're not afraid and we won't change our plans," Hallgrimsson told journalists.
Nigeria lost 2-0 to Croatia in their opening Group D game of the World Cup.
The 2018 FIFA World Cup kicked off on June 14 and will continue until July 15, with the matches being played in 11 cities across Russia.