Wakui pitched for the Japanese national team in the 2008 Beijing Olympics and 2009 World Baseball Classic.

There, he played in the 75th National High School Baseball Invitational Tournament in 2003 alongside then-ace and current Chiba Lotte Marines left-hander Yoshihisa Naruse (who was a year older than Wakui) in the spring of his second year (the equivalent of eleventh grade in the United States).

Wakui was chosen to start in the tournament finals despite having pitched only in relief up until then, but gave up six runs in just 32⁄3 innings to Koryo High School, who, led by current Yomiuri Giants pitcher Kentaro Nishimura, won in a 15–3 rout.

Wakui was picked in the first round of the 2004 NPB amateur draft by the Seibu Lions[6] and later given the uniform number 16.

On March 26, 2006, his first start of the season, Wakui earned a win against the Orix Buffaloes with Ginjiro Sumitani behind the plate (an 18-year-old rookie catcher straight out of high school), marking the first time a pitcher and catcher both in their teens had won a game in Japanese professional baseball since Tadanori Ishii (now Takuro Ishii) and Motonobu Tanishige accomplished the feat for the Yokohama Taiyo Whales in 1989.

He went 3–1 in June, earning Pacific League Most Valuable Player honors for that month, and made his first All-Star Game appearance via manager selection.

[8] He also led both Pacific and Central leagues in innings pitched (213) and hits allowed (199) and came second to only Hokkaido Nippon-Ham Fighters ace Yu Darvish in complete games (11).

Wakui was selected to the national team to play in the Olympics, starting in the game against Chinese Taipei in the group stage on August 14[10] and holding them to one run over six innings in earning the win.

[11] He followed up the effort with a two-hit, complete game (seven innings due to mercy rule) shutout against China on August 19.

[21] Wakui joined the Chiba Lotte Marines in 2014 as a free agent after playing nine seasons with the Lions, his original team.

He signed a four-year contract worth 1.6 billion yen with the Marines, who hoped to bolster their rotation with his experience and leadership.

In his final season with the Marines in 2019, Wakui struggled with inconsistency and injuries, finishing with a 7-11 record and a 4.47 ERA in 23 games.

While his mechanics are well-balanced and mostly conventional, one unique trait in his delivery is how he completely straightens his left leg in a diagonal direction after raising it (and before dropping and driving towards the plate).

[26] However, he complements it with a wide assortment of secondary pitches, including a slider, cutter, curveball, splitter, changeup, and a solid shuuto (two-seamer/sinker).