"He is getting better every day. He is good hands. I am not going to hide that it is very unlikely that he will play," Iceland coach Heimir Hallgrimsson told reporters on Thursday ahead of the Group D game.
Gudmundsson, who plays for Burnley in England's Premier League, has 65 caps for Iceland and is a key part of the team's fast, counter-attacking style.
But he was substituted with an injury midway through the second half of Iceland's impressive 1-1 draw with twice world champions Argentina on Saturday.
That surprise result kept Iceland in the frame to make the knock-out stages, having similarly shocked at Euro 2016.
Hallgrimsson said the absence of Gudmundsson -- replaced on Saturday by Rurik Gislason who plays for SV Sandhausen in Germany's second division -- would not upset the balance of his team.
"We are not afraid. It will not change the plans that we have decided before this game," he said.
Iceland playmaker Gylfi Sigurdsson, who recovered from injury in time for the World Cup and played against Argentina, is 100 percent fit, he said.
Hallgrimsson said the heat of Volgograd -- where temperatures are typically a lot higher than most other World Cup venues -- might represent an advantage for Iceland's opponents on Friday.
"It's probably better for Nigeria than Iceland to play in these temperatures," he said. "We will think about that when we do the plan. Everyone has to do it in 30 degrees Celsius (86°F)so we just have to do it like everyone else."
Hallgrimsson noted the pace of Nigeria and the aerial prowess of their striker Odion Ighalo. He also praised the development of the Super Eagles under German coach Gernot Rohr.
"I have to give credit to the coach," he said. "If you watched their games throughout the last year you can see improvement in their tactical awareness."
However, while Nigeria's players have more experience of playing in Europe's top leagues, Iceland will rely again on the strength of their collective bond which has helped them overcome far bigger rivals.
"Once you do it as a team, you’ve got a chance," captain Aron Gunnarsson said. "If you do it as an individual you won’t win anything."
(Reporting by William Schomberg, editing by Ed Osmond)
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