Mitoma missed the cut after medical staff decided he did not have enough time to recover from a hamstring injury he suffered during Brighton's 3-0 win over Wolves in the Premier League last weekend.
The 28-year-old is one of Japan's best players and scored the winner in their 1-0 friendly victory over England at Wembley in March.
Japan have been drawn with the Netherlands, Sweden and Tunisia in Group F and they start their campaign against the Dutch in Arlington, Texas on June 14th.
Moriyasu became teary-eyed as he discussed the players missing from his 26-man squad in front of around 300 reporters in Tokyo, but said they have enough strength in depth to cope.
"I think everyone who has watched Japan play can see that Mitoma is a major presence in the team," he said.
"But last year we played Brazil in a friendly and we beat them for the first time, and Mitoma wasn't available to play in that game.
"That reflects the team concept, that anyone can come into the lineup and the team still performs."
Mitoma played a key role in Japan's shock wins over Germany and Spain at the 2022 World Cup in Qatar, where they eventually lost on penalties to Croatia in the last 16.
"This will hurt him more than it hurts anyone," Moriyasu said.
"I hope once he gets over this, he will recover as quickly as possible and get back to playing with total freedom."
Monaco forward Takumi Minamino also missed out after tearing his anterior cruciate ligament in his left knee in late December.
Endo, Kamada in
Ajax's Takehiro Tomiyasu made the squad despite not having played for Japan since June 2024 because of injuries.
Crystal Palace's Daichi Kamada, Liverpool's Wataru Endo and Real Sociedad's Takefusa Kubo were all included.
Wolfsburg's Kento Shiogai, who won his first cap in Japan's 1-0 win over Scotland in Glasgow in March, also made the cut.
Defender Yuto Nagatomo was included for what will be his fifth World Cup at the age of 39.
Moriyasu has said Japan are aiming to win the World Cup and he was in no mood to change his mind despite Mitoma's absence.
"The World Cup is a special stage but that doesn't mean we'll do things any differently," he said.
"I believe the World Cup is an extension of the process we've followed up to now, and I tell the players that the process is what matters most."
Squad
Goalkeepers: Zion Suzuki (Parma/ITA), Keisuke Osako (Sanfrecce Hiroshima), Tomoki Hayakawa (Kashima Antlers)
Defenders: Yuto Nagatomo (FC Tokyo), Shogo Taniguchi (Sint-Truiden/BEL), Ko Itakura (Ajax/NED), Tsuyoshi Watanabe (Feyenoord/NED), Takehiro Tomiyasu (Ajax/NED), Hiroki Ito (Bayern Munich/GER), Ayumu Seko (Le Havre/FRA), Yukinari Sugawara (Werder Bremen/GER), Junnosuke Suzuki (Copenhagen/DEN)
Midfielders/forwards: Wataru Endo (Liverpool/ENG), Junya Ito (Genk/BEL), Daichi Kamada (Crystal Palace/GER), Koki Ogawa (NEC/NED), Daizen Maeda (Celtic/SCO), Ritsu Doan (Eintracht Frankfurt/GER), Ayase Ueda (Feyenoord/NED), Ao Tanaka (Leeds United/ENG), Keito Nakamura (Reims/FRA), Kaishu Sano (Mainz/GER), Takefusa Kubo (Real Sociedad/ESP), Yuito Suzuki (Freiburg/GER), Kento Shiogai (Wolfsburg/GER), Keisuke Goto (Sint-Truiden/BEL)