Sherdog first reported the matchup yesterday.
Day was forced to withdraw from his scheduled bout against Lambert after suffering elbow and biceps injury that will sideline the Canadian middleweight for an unknown period of time.
MacDonald (20-10) opened his UFC career with back-to-back wins over Ed Herman and Chris Leben but then dropped three of his next five bouts to Rich Franklin, Yushin Okami, and Maia. He’s also earned UFC victories over Rory Singer and Joe Doerksen.
Following three-straight victories over Rob MacDonald, Terry Martin, and Brenden Lee Hinkle to open his UFC career Lambert was defeated by Rashad Evans then posted a knockout victory over Renato “Babalu” Sobral. He next lost back-to-back light heavyweight bouts to Wilson Gouveia and, most-recently at UFC 85 in June, Luis Cane, which forced him drop down to the 185-pound middleweight division.
UFC 88 takes place on September 6th at the Phillips Arena in Atlanta, Georgia and features a main event light heavyweight matchup between former 205-pound champion Chuck Liddell and “The Ultimate Fighter 2″ winner Rashad Evans.