The 30-year-old striker has struggled for first-team starts since moving on a free transfer from Werder Bremen in the close season.
But Olsson believes the ex-Ajax forward will find his best form as the season progresses.
"He's still settling in but he knows what it's about – he's been a professional player for 12 or 13 years," said Olsson. "He's disappointed not to be starting but the competition we have up front is our greatest strength.
"When he's come on a few times, especially at Newcastle, he's shown he's a good player. When he gets his chance he will do well. He's a friend of mine and I've told him this league, around Christmas and December, everyone will get their chance. He will take it when he gets that chance.
"It's a matter of us performing well and that can make it difficult for some players. He came during the final weeks of the summer transfer window and had no club during the summer so he had no proper pre-season.
"That prevented him from playing too much in the beginning and now we're playing well."
Rosenberg has been kept on the sidelines by the form of Shane Long and Romelu Lukaku, who were paired together for the first time in Saturday's 2-1 win at Wigan.
Lukaku's season-long loan from Chelsea includes a recall option in the January transfer window.
But Blues boss and former Albion chief Roberto Di Matteo is remaining non-committal on the future of Lukaku, who is ineligible to play against the Blues at The Hawthorns on Saturday.
The Italian said: "We'll have to see what's in the best interests of the club and the player."