Both repeatedly won the most prestigious heavyweight boxing championships, such as WBC, IBF, and WBO.
Purdy, ranked No. 4 by the IBF, accepted the bout on four weeks' notice.
One-time IBF super-featherweight title holder St Clair, 32, has never been stopped in his 46-fight career.
Morales won with a unanimous decision against the El Salvador fighter, who held the IBF title.
Hatton relinquished his WBA welterweight title on Thursday, having given up the IBF light welterweight crown in April.
Philadelphian Cunningham, 30, won the IBF title on a split decision from Krzysztof Wlodarczyk in Poland last month.
The former IBF Cruiserweight Champion said he has no interest in dropping back down to that weight class.
Hopkins scored a 12-round unanimous decision over Tavoris Cloud to claim the IBF light heavyweight championship Saturday night.
Vitali Klitschko is the current WBC heavyweight champion, while his younger brother Wladimir owns the IBF and WBO belts.
On the same bill, Mexico's Rafael Marquez retained his IBF bantamweight title with a third-round knockout of Heriberto Ruiz.
The Danish-based boxer was previously rated in the WBC's top-10 middleweights and is the current IBF international super middleweight champion.
Welshman Maccarinelli had targeted another Don King fighter, IBF champion Steve Cunningham, for the rearranged date at the Cardiff International Arena.
Dereck Chisora's bout against IBF and WBO heavyweight champion Wladimir Klitshcko will be shown on a big screen in the 11, 000-capacity arena.
Hatton's last fight for Warren was on 6 April 2005 when he notched a career-defining win over IBF light-welterweight king Kostya Tszyu.
Haye holds the WBA version of the heavyweight crown, while the Klitschko brothers between them hold the WBC, IBF and WBO belts.
The world number one is currently 36-year-old American Nate Campbell, who recently beat Juan Diaz for the WBA, WBO and IBF titles.
The Bolton-born fighter is currently preparing for the defence of his WBA light-welterweight title against IBF champion Zab Judah on 23 July.
It was thought that Johnson, 37, might be dropped as the IBF's mandatory challenger after apparently choosing to fight Calzaghe over Woods.
An impressive display could also open the door to potential unification fights with IBF champion Lucian Bute and WBA holder Mikkel Kessler.
The meeting with Haye will be the seventh defence of his IBF title, which he won against Chris Byrd in April 2006.
Hobson guided Sheffield's Clinton Woods to the IBF light-heavyweight title and also promoted some of Ricky Hatton's fights in the United Kingdom.
The recognised world titles at the weight are held by Jean Pascal (WBC), Tavoris Cloud (IBF), Gabriel Campillo (WBA) and Juergen Braehmer (WBO).
BBC: Nathan Cleverly helped to title by leaving Calzaghe gym
The next evening in Washington, he'll defend his WBA and IBF light-welterweight world titles against Lamont Peterson in a marquee fight on HBO.
Also on the undercard, Uganda's Kassim Ouma won the IBF light middleweight crown over Verno Phillips following a unanimous decision by the judges.
Hobson, who has signed a three-fight deal with American television network HBO, had hoped to announce an opponent for IBF and WBA title-holder Hatton last week.
Under IBF regulations, Englishman had two hours to take off the extra pound, but still weighed 147.8 when he stepped back on the scale.
After IBF and WBO champion Wladimir Klitschko pulled out of a title defence against Derek Chisora, his US advisors met with Haye's American advisors.
His next contest is set for 6 January, when he will attempt to regain the IBF light welterweight belt from current holder Juan Urango.
In March 2006 Calzaghe fought through the pain of a hand injury to demolish Jeff Lacy to unify the IBF and his WBO crown.
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