Liverpool paid $9.8 million for the 24-year-old, who becomes the first Japanese player for the club.
Both Barcelona and Dortmund can qualify for the round of 16 of the Champions League with a win on Wednesday.
The Spanish giants can qualify for the round of 16 with a win against PSG, who needs to avoid defeat to top Group A.
The Serie A giants will top Group D with a win on Tuesday, while Atletico will qualify if it wins in Turin.
Juventus could secure qualification to the round of 16 with a win in Moscow against Lokomotiv on Wednesday.
Los Blancos can take a major step towards qualification with a home win against Galatasaray on Wednesday.
Barcelona hopes to make it three wins in four games in Group F when it hosts Slavia Prague on Tuesday afternoon.
The five-time European Cup winners look to extend their lead at the top of Group F when they take on Slavia Prague.
Real Madrid faces a tricky trip to Istanbul to face Galatasaray, knowing that a defeat could make qualification from Group A very difficult.
The Serie A giants look for a second win in three games in Group D when they host the leaders of the Russian league on Tuesday.
Barcelona hosts Inter Milan on Wednesday with both teams looking for a first win in the Champions League this season.
Two of the Champions League dark horses meet on Tuesday afternoon when Atletico Madrid hosts Juventus.
Two of the pre-tournament favorite meet in Paris as PSG hosts Real Madrid, with both teams depleted by injuries and suspensions.
Lionel Messi could make his first appearance of the season when Barcelona travels to Germany to face Dortmund on Tuesday.
Liverpool begins its quest for a second consecutive Champions League crown on Tuesday when it takes on Napoli.
Manchester City is the favorite to win the tournament for the first time, with Liverpool and Barcelona close seconds.
Liverpool looks to become only the second team to defend its Champions League title when the tournament kicks off on Tuesday.
Liverpool will hope to win a sixth European Cup when it faces Real Madrid on May 26.
Ronaldo almost single-handedly saw off Atletico Madrid.
Dortmund's team bus was attacked on Tuesday evening.
Kylian Mbappe is one of Europe's most talked-about strikers.
The competition has been screened exclusively on BT Sport since 2015.
Arsenal has not made it past this stage in six years.
Guardiola has experienced a difficult first winter in the Premier League.
The Manchester United manager needs a replacement for the departing Eric Bailly.