In 2012, Miyaichi spent loan periods at fellow Premier League clubs, Bolton Wanderers and Wigan Athletic, helping win the FA Cup with the latter in 2013.

On 5 February, in his first appearance for Feyenoord, he played the full 90 minutes against Vitesse Arnhem in a 1–1 draw, and was named man of the match.

[9] Miyaichi then played against Heracles Almelo where he scored the opening goal and created the second in a 2–1 league win for Feyenoord.

Miyaichi made his pre-season debut against Malaysia All-Stars XI, he started the match before being substituted for Robin van Persie in the 66th minute.

[19] Shortly after arriving at Bolton, manager Owen Coyle spoke of his admiration for the young player, even though Miyaichi had yet to play in a competitive match for the team.

[29][30] On 9 March, he made his return as a substitute for Callum McManaman in a 3–0 FA Cup quarter final win away to Everton.

[35][36][32] Despite interest from former club Feynoord and SC Heerenveen, upon 1 September 2014 Miyaichi was sent on a season long loan to fellow Dutch side FC Twente.

[41] Miyaichi's transfer was confirmed on the club's website the next day, where it was revealed that he would wear the number 13 shirt for the upcoming season.

[43] In preparations for the 2017-18 season, Miyaichi had another major set back, suffering from another cruciate ligament tear, this time to his right knee.

On 5 July 2021, after having spent his entire senior career in Europe, Miyaichi returned to his native Japan, signing for Yokohama F. Marinos in the J1 League.

[47] Miyaichi was assigned the number 17 shirt and on 18 September 2021, he made his debut against Nagoya Grampus, coming on as a substitute in the 55th minute.

[52] On 2 July 2022, Miyaichi scored a late goal against Shimizu S-Pulse after coming on as a substitute, as Yokohama secured a 5-3 away victory.

[53] Four days later, Miyaichi made an impact off the bench again, scoring another late goal against Sanfrecce Hiroshima in a match that ended 3-0 to Yokohama.

[54] Miyaichi's performances during the season saw him earn a re-call to the Japan national team after almost a decade since his last international appearance in 2012.

[55] Miyaichi suffered another anterior cruciate ligament rupture in his right knee during Japan's final EAFF E-1 Football Championship match against South Korea and will miss out the rest of the season following surgery.

[61][62] On 10 June, Miyaichi scored his first league goal of the 2023 season against Kashiwa Reysol in the 97th minute to win the match 4-3 after coming on as a substitute.

[63] A technically gifted player, Miyaichi plays as a winger and has been noted for his blistering pace and ability to run and dribble past defenders.

"[64] Former Feyenoord director, Eric Gudde praised Miyaichi saying that "He is a player with good pace, dribbling and a lot of tricks to get behind the opponent.

Miyaichi received his first call up to the senior team for the 2014 World Cup qualification against Uzbekistan in February 2012, though he didn't feature in the game.