Brought on as a second-half substitute with Javi Gracia's men 2-0 down, the Spaniard channeled his disappointment into a stellar display, scoring an audacious curling equaliser before sealing a brace and victory for the Hornets in extra-time.
Deulofeu chose the biggest stage to showcase his undeniable talent. Now he must prove that he is too good a player, too important to Watford, to be omitted from the line-up in such a key fixture again.
Doing so will not be achieved by eye-catching cameos, though. Rather Deulofeu has to discover something that has eluded him all his career: consistency.
A winger who could play on either flank, possessed pace, cunning and an encouraging work ethic, he appeared destined for stardom at the Camp Nou.
“Who stood out from my Barça B team? Deulofeu is the stand out,” said current Spain manager Luis Enrique in 2014, reflecting on his time in charge of Barcelona's second string. “He’s got speed, he can beat a player and score goals.”
Deulofeu penned professional terms with Barça shortly after his 19th birthday, reward for a season which saw him net 18 times for the B team. But first-team opportunities were in short supply, making just two La Liga appearances before being sent out on loan to Everton and then Sevilla.
His talent was obvious during his first season in the Premier League, and Everton took the chance to sign Deulofeu permanently after his loan spell with Sevilla. But then came the downturn, a drop-off in form seeing him fall out of the side and loaned to AC Milan.
A second champ at the Camp Nou came in the summer of 2017, with the La Liga giants exercising a buyback option on the player; any plans to then immediately sell him on for a profit were shelved in light of Neymar's shock exit for Paris Saint-Germain.
So Deulofeu stayed and he played, starting the season on the right flank before eventually being usurped by new arrivals Ousmane Dembélé and January signing Philippe Coutinho.
By January of 2018, he was on the move again, joining Watford on loan – the fourth of his career – ahead of a permanent deal last summer.
Deulofeu's great problem has always been that he is desperately inconsistent. He has a habit of starting brilliantly at a new club before tailing off. At 25, he is no longer a young La Masia graduate with the world at his feet; it's time for him to deliver.
His two-goal showing to rescue Watford against Wolves and send the Hornets through to the FA Cup final was spectacular. It will mean he is remembered as an icon at Vicarage Road for years to come and likely see him relied upon as a key player for the remainder of the season.
And there have been flashes of his brilliance throughout his Premier League return, such as the hat-trick against Cardiff City and a handful of remarkable goals.
But if he wants to avoid being benched for the biggest games in future, Deulofeu needs to find a level of consistency. The semi-final – the disappointment of being left out and the ultimate joy – has to be his springboard.