USA Today had earlier reported the news on its website.
UFC has yet to officially announce the card or a location for the event, which is expected to air on Fox.
Henderson (17-2) defeated former 155-pound champion Frankie Edgar by split decision in a rematch on Aug. 11 to retain his title.
Diaz, who has competed at 170 pounds, is 3-0 since returning to lightweight. He secured a title shot with a second-round submission May 5 of Jim Miller.
"The Ultimate Fighter 5" winner, Diaz has a professional mixed martial arts record of 16-7.
Penn and MacDonald have been locked in a war of words for several months.
They were originally booked to resolve their dispute Sept. 22 at UFC 152 in Toronto. But the fight was postponed when MacDonald suffered a cut above his right eye during training camp.
More than 30 stitches were necessary to close the wound.
Penn, a former UFC champion at welterweight and lightweight, seriously considered retiring after a lopsided unanimous-decision loss to Nick Diaz on Oct. 29, 2011.
After softening his stance on retirement, the 33-year-old Penn received a challenge from MacDonald and he accepted it.
MacDonald (13-1) is ranked No. 5 among welterweights by ESPN.com. His lone loss, a third-round knockout, came on June 12, 2010, to current interim titleholder Carlos Condit.
Since that setback, MacDonald has reeled off three wins in a row.
Henderson will attempt to lift the light heavyweight title belt from Jones on Sept. 1.
Rua (21-6) is itching for another shot at Jones. It was Jones who took the 205-pound belt from Rua, via third-round TKO, in March 2011.
Gustafsson is 14-1. He has won five in a row since losing by submission to fellow light heavyweight contender Phil Davis in April 2010.