As the Dutch giants ran riot in Portugal, the Ivorian scored twice each side of half-time, and his Ajax performances continue to astound puzzled Hammers fans.
Haller, 27, was a complete disaster in the London Stadium, but he has made a quick transition to Holland.
And his most recent performance put him in the record books.
His four goals made him only the second player in Champions League history to score four goals in his debut, and the first to do it in an away game.
Within nine minutes, Haller had doubled Ajax’s lead, stooping to head in a rebound from close range then diving to stab home a low cross from six yards.
On 33 minutes, Paulinho equalized for the hosts, before Steven Berghuis gave the Dutch a 3-1 half-time lead six minutes before the break.
Haller’s four-goal haul was completed in less than 20 minutes of the second half.