The winger looked in great shape after going on as substitute in the 2-1 win over Liverpool.
He had been sidelined by a painful ankle, missing the trip to Sunderland, but made a big impact against the champions.
He now teams up with his national team for friendlies at Hampden Park on Saturday and Wembley on Tuesday.
Albion head coach Fabian Hurzeler said: “We had an exchange with the Japanese medical department, so they know how much he trained in the last week.
“They know where he is at the moment.
“I hope it doesn't only count for Japan but for all the national teams, that they take care of our players because I think we give them in really good shape to them and hopefully they come back in good shape.
“Therefore, everyone has responsibility now.”
Mitoma is closing in on his second World Cup and is the one member of Albion’s eight-strong representation at the 2022 finals who remains with the club.
He will be keen to return to international action having not played for Japan since early September due to an ankle injury.
Mitoma said: "We want to see how well what we've been doing so far will work against European teams, and it's also important to see how well we can perform what we have been doing, even with changes in the players.
“We will be looking at each player's condition, individual ability, and whether they can win duels.
"I think it's great that we have a variety of players joining us, creating good competition. Each player has different strengths, so I'm hoping to learn from their strengths and then use that knowledge to my advantage when I get the chance to play."