Midfield Mulumbu has added bite to Albion's line-up and given protection to a back four which was often left horribly exposed in the top flight under the ill-fated tenure of Tony Mowbray two seasons ago.
The Baggies have taken the Premier League by storm, winning 3-2 at Arsenal and collecting 11 points from their first seven league games to lie sixth in the table.
Olsson told the Express & Star: "Youssouf is a massive player for us. He has been putting in great performances ever since he came to this club. He's a top guy and a funny guy and we're happy with the way he has been playing for us.
"For a centre-half, being protected like that makes it so much easier for you.
"Maybe that's what we lacked last time we were up. We had good players in midfield but no one really covering us.
"With Youssouf and Paul Scharner being a bit more defensive in their approach, it's a big help."
Olsson feels there is more top-flight experience and know-how in the team thanks to the likes of Scharner and Steven Reid being at The Hawthorns.
And he added: "Paul's a big help. Youssouf is an intelligent player and his decision-making is good and playing with Paul can only help him with his experience.
"It is the same with Steven Reid and other experienced players."