The Gers sacked former manager Pedro Caixinha back in October but are yet to appoint a permanent successor, with interim manager Graeme Murty likely to remain in control for his seventh game as caretaker boss on Saturday.
Aberdeen released a statement on their official website this week, informing fans that they had rejected an approach from the Gers for McInnes - but it is becoming more and more inevitable, according to Nicholas, that McInnes will take over.
Talking to Sky Sports, Nicholas ridiculed Rangers by criticising their patience over the deal, before stating that McInnes' arrival will make Rangers stronger than Aberdeen. He also outlined the need to invest at Ibrox, describing the current Rangers squad as "pretty average".
"It's a done deal isn't it, it's a matter of time before McInnes left Aberdeen," Nicholas said. "Rangers have waited so long to pay compensation, why does it take so long? If Rangers want you, they should have made a move. It's left him isolated.
"I always though McInnes was coming to his sell-by date at Aberdeen. He can't take them any further. I think this will happen now, but what will happen is that it will make Rangers slightly stronger and Aberdeen weaker.
"Other than that, he needs funds. The team is pretty average. They're not even close to Celtic, no matter who they have in charge - Caixinha or McInnes. Rangers don't have a big financial pot, and it won't be until the summer until they start to build."
Interim manager Murty has taken control of the Gers last six games, winning four of those including two wins in his last two games against the Dons which has seen Rangers move into second place.
It remains to be seen if McInnes will be appointed at Ibrox this week, as Rangers prepare to face Ross County on Saturday at Ibrox before a difficult away trip to Neil Lennon's Hibernian on Wednesday evening next week.