They will play the victor of Wednesday's other semi-final between Ajax Amsterdam and Tottenham Hotspur.
Robertson's injury meant he did not return to the pitch for the second half - Wijnaldum replaced the Scot and Milner moved to left-back.
Jose Mourinho attributes Liverpool's stunning victory over Barcelona to manager Jurgen Klopp.
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.
The goals from Wijnaldum were shocking to say the least, and gave Liverpool new life.
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"When they scored the goal, when they were pressing so high, it was really hard".
Liverpool have reluctantly sold Luis Suarez and Philippe Coutinho to Barca in recent years, but Johnston feels the club should now be trying to lure the likes of Messi to Merseyside.
Late winners from the Belgium worldwide against Everton and Newcastle - the latter of which was provided by Shaqiri's free-kick - have kept Liverpool in the title race, but it's in the Champions League now that the pair are now needed.
Once again the Catalan giants, who last won the Champions League in 2015, wilted when the stakes were highest.
People were in utter disbelief of the goal, as Barcelona continued to struggle against a stubborn Liverpool team. Ernesto Valverde must have been furious with his side.
Asked about the set-piece at full-time, the Liverpool defender told BT Sport: "It was just instinctive".
But that alone was no excuse for the timid way Barca allowed themselves to be ripped apart by Liverpool in one of the most wretched results in the club's history.