Nolan insists Brentford striker Yoane Wissa would not be an adequate replacement for the Swede after Newcastle had a £25m offer rejected on Monday.
"If you got the majority of Newcastle fans on, they'd be saying we want to keep Isak, we need to pay him the money," he told talkSPORT.
"Are you going to find another Isak? There's talk about Wissa, but I'm not too sure that he gets the goals and becomes that No.9.
"It's different, there's no disrespect to Brentford here, but it's different playing for Newcastle and playing as a No.9.
"There's an expectation there, it's a big difference."
Nolan added: "Wissa will be going in to take the No.9 shirt, which has a huge history behind it.
"He has to be playing alongside Isak if he joins.
"If they lose Isak, that's a major blow to Newcastle and where they're trying to go in the Premier League."