By Keith Idec

Kell Brook’s supporters assured Errol Spence Jr. that he’s not ready for this.

They reminded the undefeated welterweight contender that Brook by far is the top opponent of his four-year pro career, unlike anyone Spence has faced to date. Spence (21-0, 18 KOs) and his trainer, Derrick James, are undeterred with his first world title fight just a day away.

“The guy said before he’s too green,” James said during a press conference Thursday in Sheffield, England. “It doesn’t matter who you’ve fought before this point, because who did Leon Spinks beat before he fought Muhammad Ali? Nobody.”

Unlike Spinks, a decided underdog who was 6-0-1 (5 KOs) when he upset Ali by split decision in February 1978 to win the WBA and WBC heavyweight titles, Spence is slightly more than a 2-1 favorite over the defending champion. Brook’s trainer, Dominic Ingle, doesn’t understand why handicappers have installed his fighter as the underdog.

The 31-year-old Brook (36-1, 25 KOs) is the more experienced, proven fighter. Their scheduled 12-round fight for Brook’s IBF welterweight title also will be contested in Brook’s hometown of Sheffield.

“Let’s not forget that when Leon Spinks beat Muhammad Ali, Muhammad Ali was way, way, way past his best,” Ingle said, referring to the fact that Ali was 36 when a 24-year-old Spinks upset him. “So if that’s the best analogy [you’ve got], we haven’t got a problem on Saturday night.”

Former WBO junior welterweight champion Chris Algieri (21-3, 8 KOs) is the most accomplished opponent the 27-year-old Spence has beaten in his first 21 fights. Spence steamrolled Algieri on his way to a fifth-round technical knockout 13 months ago at Brooklyn’s Barclays Center, but Brook is undefeated at welterweight.

His lone loss came in a middleweight title fight against Gennady Golovkin (37-0, 33 KOs), who beat Brook by fifth-round TKO September 10 at O2 Arena in London. Ingle stopped that bout because Brook had suffered a fractured right orbital bone earlier in the fight and he didn’t want Golovkin to inflict permanent damage.

Ingle doesn’t expect Brook to have any such problems against Spence, a heavily hyped southpaw who has knocked out 86 percent of his opponents.

“Kell Brook’s always talked about his dream being to fight here at Bramall Lane,” Ingle said, referring to the soccer stadium where the fight will take place. “But one man’s dream is another man’s nightmare. And on Saturday night, it’s gonna be Errol Spence’s nightmare. It’s not gonna be a problem. You know, he’s come here with a lot of confidence into this fight. But to me, it just seems like they’ve pushed him from America to come over.

“And it’s almost like when you’re at school, and there’s a kid that nobody wants to fight. And they all push him. ‘You have a fight with him. You go first and we’ll all see what happens then.’ And it’s just like he’s the guy who’s brave enough to do it, but not really qualified to do it. There’s a big difference in that.”

The outspoken Ingle also questioned Spence’s pedigree by demeaning the DeSoto, Texas, native’s failure to win even a bronze medal at the 2012 Summer Olympics in London.

“Danny Garcia won’t come here,” Ingle said. “Keith Thurman won’t come here. The only guy who’s fought Kell Brook and put in a very good performance was Shawn Porter, and Shawn Porter’s there [points to him]. He’s a proven fighter. And ever since getting beaten by Kell Brook [by majority decision], he’s shown how good he is because he’s gone in with other guys and done well. But what I’m saying to you is this – calling Errol Spence an Olympian, you’ve got to win a gold, silver or bronze to be an Olympian. … I’ve had guys in Olympic qualifiers, but they’re not Olympians. So if that’s the best he’s got, that’s all right. No problem.”

A crowd of roughly 27,000 is expected Saturday at Bramall Lane for the card headlined by Brook-Spence.

Showtime will broadcast Brook-Spence in the United States, starting at 5:15 p.m. ET/2:15 p.m. PT. Sky Sports Box Office is set to air their welterweight championship showdown as the main event of a pay-per-view broadcast in the United Kingdom (6 p.m. BST; £16.95 in HD).

Keith Idec is a senior writer/columnist for BoxingScene.com. He can be reached on Twitter @Idecboxing.