However, that substitution proved to be a blessing in disguise for the Merseysiders, as Wijnaldum scored two quick-fire goals to level the tie on aggregate.
Liverpool will play the victor of the other semi-final on Wednesday between Ajax and Tottenham Hotspur, with the Dutch side leading 1-0 from the first leg in London.
For Barca, who went out on away goals to AS Roma in the quarterfinals past year after winning the first leg 4-1, it was a bitter night that will raise many questions for coach Ernesto Valverde.
But the Reds did not have to wait long to turn hope into opportunity.
So scroll and click through the slides to read the player ratings from the game.
"We should be buying Lionel Messi!".
Disney schedules new 'Star Wars' movies; more
While we aren't 100 percent sure what's coming after Star Wars: The Rise of Skywalker , the picture is slightly more clear. Weiss, the creators of HBO´s " Game of Thrones ", and a separate trilogy by " Last Jedi " director Rian Johnson .
The astonishing revival will now forever be mentioned in the same awed tones as Liverpool's never-to-be-forgotten "Miracle of Istanbul" when Steven Gerrard and company recovered from 3-0 down at half-time to beat AC Milan on penalties in the 2005 Champions League final.
Ter Stegen was next called into action to block an Andy Robertson blast from the angle before the Reds left-back followed Henderson in shaking off a knock to continue.
"In the first half, we had options to score". Lovely play by Barcelona and Messi's strike was tipped over the bar by Alisson after being played in by Alba. Georginio Wijnaldum came on the pitch in his place.
From a corner, as Barcelona players turned away and went to prepare themselves, Alexander-Arnold and Origi were the only ones alert as the 20-year-old swiftly stroked the ball low into the box.
It was Alexander-Arnold that delivered the clever corner kick from which Origi swept home their fourth goal.
Two minutes later, he made another run into the area and met a cross from Xherdan Shaqiri with a firm header into the corner past a flat-footed Ter Stegen.
Liverpool risked finishing the season without a trophy with Manchester City leading the Premier League title race with one game to go and Barcelona heavy favourites to advance to the final after a commanding win at the Nou Camp.