He has recalled nine players who remained in Fiji after they beat the Maori All Blacks in Suva last week, and retained 18 players from the squad who lost the return match in Rotorua on Saturday.
"The players who remained in Fiji last week underwent specific physical conditioning so I expect them to come fitter and fresh this week," McKee said.
"We expect Japan to play a very up-tempo game. Once again we will have to work very hard in our defensive system and look to dominate the physical collisions."
Only five of the squad are Fiji-based.
McKee said the home and away matches against the Maori All Blacks were positive for the team despite the series being drawn.
"There have been some good learnings from our two matches against the Maori All Blacks and players have benefited from the physical matches over the past two weeks," he said.
However, there was no room for Toulon wing Josua Tuisova, who played in the opening match of the series when Fiji beat the Maori All Blacks for the first time in 62 years, while Bordeaux's Semi Radradra - a key member of the side that beat France in Paris last November - also missed the cut.
Forwards: Lee-Roy Atalifo (Jersey Reds), Campese Ma’afu (Leicester Tigers), Eroni Mawi (local), Peni Ravai (Bordeaux-Begles), Manasa Saulo (London Irish), Kalivati Tawake (local), Mesulame Dolokoto (local), Ratu Vere Vugakoto (local), Sam Matavesi (Toulouse), Leone Nakarawa (Racing 92), Apisalome Ratuniyarawa (Northampton Saints), Albert Tuisue (London Irish), Semi Kunatani (Harlequins), Viliame Mata (Edinburgh), Dominiko Waqaniburotu (Brive), Peceli Yato (Clermont)
Backs: Frank Lomani (local), Henry Seniloli (Doncaster Knights), Alivereti Veitokani (London Irish), Ben Volavola (Racing 92), Levani Botia (La Rochelle), Waisea Nayacalevu (Stade Francais), Jale Vatubua (Pau), Filipo Nakosi (Toulon), Patrick Osbourne (Kubota Spears), Josh Matavesi (Newcastle Falcons), Kini Murimurivalu (La Rochelle)