Paul Butler was ready to race straight to the top the moment he was able to secure even the slightest amount of leverage.

For the two-time bantamweight titlist, it came in the immediate aftermath of Naoya Inoue’s repeat win over Nonito Donaire in their lineal/WBA/WBC/IBF unification clash this past June 7 in Saitama, Japan. Inoue (23-0, 20KOs) was emphatic in his desire to remain at bantamweight just long enough for a shot at becoming his nation’s first-ever undisputed champion.

Enter Butler (34-2, 15KOs), who holds the WBO title—the last piece of the puzzle, which was enough to spark a conversation. It was a no-brainer to accept the dangerous but lucrative and potentially rewarding road assignment to face Inoue in their four-belt and lineal unification bout on Tuesday at Ariake Arena in Tokyo (ESPN+, 2:30 a.m. ET).

“The chance to unify against Inoue was the fight I wanted before anything else,” Butler told BoxingScene.com. “It was never a matter of saying yes or no. It was just a matter of when the fight would happen.

“December was proposed to us and we were fine with that.”

For Butler, it was his patience rewarded after enduring two separate failed bids at securing a mandatory title shot versus then-WBO titlist John Riel Casimero. The two were due to meet last December in Dubai, only for Casimero to pull up ill on the morning of the weigh-in as he withdrew from the fight. The brash Filipino was mandated by the WBO to immediately reschedule the fight to avoid being stripped of the title.

The bout was back on the calendar for April 22 in Liverpool, only for Casimero to violate BBBofC rules by using a sauna during fight week. The incident left him removed from the fight, with Butler left to contend for the interim WBO title which he claimed in a twelve-round win over late replacement Jonas Sultan. The 34-year-old Brit was elevated to full titlist on May 4, after Casimero was relieved of the crown for failure to defend.

It wasn’t the way that Butler envisioned becoming a two-time bantamweight titlist, though he simply played the hand he was dealt. The pot of gold waiting at the end of the rainbow justified the frustrating journey leading to this point. The winner of Inoue-Donaire would be left with all but one bantamweight title, with both having already expressed a willingness to next face Butler. Inoue prevailed via second-round knockout, after which point he set his sights on Butler who was waiting by the phone and ready to accept the challenge.

“When I boxed in April, Inoue and Donaire were scheduled for their rematch. So, we always knew there was a chance to get this fight,” noted Butler. “We discussed the possibility of making a title defense in October if this fight couldn’t happen.

“But once Inoue won, we were able to negotiate straight away and get this done. I couldn’t be happier.”

Jake Donovan is a senior writer for BoxingScene.com. Twitter: @JakeNDaBox