Kirk Johnson won't get his chance at Lennox Lewis. At least not yet.

A torn chest muscle suffered during a sparring sesion has forced Johnson to call off his June 21 bout against the heavyweight champ. The results of an MRI Thursday night in Houston revealed the injury, which will sideline Johnson from 4-6 weeks.

But that doesn't mean the card at the Staples Center in Los Angeles is off, despite very slow ticket sales.

Lewis was reportedly in his finest shape since his first meeting with Evander Holyfield and may not want to waste a training camp in the Poconos.

Fight spokesman Steve Brener said he was attempting to contact promoter Gary Shaw for a decision on whether the program will go on.

The most obvious replacement would be Vitali Klitschko, Lewis's mandatory WBC challenger, who was slated to face the undefeated Cedric Boswell on the June 21 undercard.

"I think we should fight Klitschko," Lewis' trainer, Emanuel Steward, told The Dallas Morning News. "Why not? He was supposed to be on the undercard against someone else, plus he's in training. It would eliminate a lot of problems for us."

But Klitschko is looked upon by Lewis and his HBO backers as a pay-per-view opponent and has been penciled in for a December shot with the real champion.

Chris Byrd, the IBF title-holder, has volunteered to step in and replace his buddy Johnson.

Byrd, attending the International Hall of Fame ceremonies in Canastota, N.Y., (where, ironically, Johnson's trainer, Curtis Cokes, will be inducted this weekend), said, "I haven't seriously trained in six months, but I'm never really out of shape. I'd be ready by next Saturday."

But Lewis has made it clear that he doesn't want to fight the clever southpaw, who might still fit in the mix if, as expected, Don King makes a move.

King could offer his fighter, Lamon Brewster of Los Angeles, as a substitute. Brewster has been training for a June 28 shot at the WBO belt holder, Corrie Sanders of South Africa, who is promoted by the Klitschkos' German connection, Universum.

For borrowing Brewster, King could repay Universum by putting Byrd in to fight Sanders -- thereby probably setting up a rematch with Wladimir Klitschko (for either Sanders or Byrd).

The slow ticket sales at the Staples Center -- one report indicated just about a thousand seats had been taken -- would not necessarily cancel the show. The gate has already been guaranteed by the Memphis interests who promoted Lewis's last fight, June 8, 2002, against Mike Tyson.

Byrd thought Johnson had "a heck of a chance" at upsetting Lewis, a 9-1 favorite.

But during a sparring session in Dallas, Johnson felt a pain in his chest. The MRI revealed a partial tear of the left pectoral muscle. The injury will not require surgery.

Still, it's a bitter blow to Johnson. Despite losing to John Ruiz last year, Johnson had managed to secure another big fight and was predicting that it would be an "easy" night for him against the aged champion.

"We're very disappointed about the postponement because we truly believe that Kirk had a great shot to beat Lennox Lewis and become the new heavyweight champion,'' Johnson's promoter Dino Duva said in a statement issued by Brener.

Johnson now may have to return to the end of the line since Lewis may have other plans.

The champion had agreed to fight Vitali Klitschko, his WBC mandatory, if he beat Johnson -- there was a rematch clause if he lost -- and HBO had penciled in that pay-per-view fight for December.

The network's Oct. 4 pay-per-view date may now go to the rubber match between Marco Antonio Barrera and Erik Morales, with the Roy Jones Jr.-Evander Holyfield bout pushed back to November.

Information from The Associated Press was used in this report.