Riis has scored twice in four Championship matches since arriving from Preston North End.
However, in the rest of their forward pack, Sinclair Armstrong has gone 26 games without a goal, Fally Mayulu has not scored since August last year, and Harry Cornick has only played 51 minutes in league football since the start of the last campaign.
Struber confirmed there has been interest in recent weeks from abroad in signing Armstrong and “other strikers” but that he was not leaving.
“This is not a big surprise but at the same time it is crystal clear that he [Armstrong] stays here right now,” he said.
“The window here is closed, this would mean we could not bring in a replacement.”
Armstrong has scored three goals in 43 matches since signing from QPR in July 2024, the last in a 4-0 win against Plymouth Argyle in November.
And how the 22-year-old reacts in front of goal is the biggest thing the coaches are working with him on, Struber said, to improve his goal return.
“He’s a young striker, maybe we forget this sometimes. He has to learn, he has to grow, he has potential to grow.
“We have to work with him very close, and with him we have to bring out and maximise his potential. He will stay and is a big part in our roster.”