‘Bomber’ Bellew moved up to cruiserweight from light-heavyweight, the division Spelman is looking to take over.
Having failed to capture world honours at light-heavy, he claimed the WBC world cruiserweight championship against Illunga Makabu in 2016.
That belt is just one on the line at the Manchester Arena tonight.
Usyk holds the WBC, WBA, IBF, Ring Magazine and WBO belts, and captured the Muhammad Ali Trophy earlier in the year, though that will not be on the line.
Bellew has made no secret of his intention to retire after next Saturday’s fight, and Spelman questioned whether this attitude will lead to his downfall.
“I would love Tony to do it,” Spelman said.
“But I think Usyk is a cut above.
“I don’t know what they are putting in that water, you’ve got him and (Vasily) Lomachenko there. (The pair are close friends and share a near-impenetrable style).
“You can’t write Bellew off.
“I wouldn’t bet against him.”
Bellew was an underdog going into fights against Makabu and two fights with David Haye, but came out on top in all three.
He goes into next Saturday’s fight with the World Boxing Super Series champion as the extreme underdog.
Spelman believes that, despite the odds, you shouldn’t write off the Evertonian, but questioned his will to win.
“He’s got so much heart,” he said.
“But, he knows himself, and he’s said he is looking for the big money fights.
“He cares for his pride, but he’s getting paid a lot of money for this, and he’ll probably retire after this.
“So, how much does he really want it? I know he’s a passionate man, but how much does he really want it?
“Usyk is going to go up to heavyweight, but Bellew is heading into the end of his career.
“It’s a win-win for both”
This evening’s blockbuster is a strange one, as it seems inevitable that both men will be fighting at cruiserweight for the final time.
Bellew is likely to retire, whilst Usyk has declared his intention to move up to heavyweight and challenge for world title honours there.