Barca failed to hold on to their 3-0 lead from the first leg as they fell to a shocking 4-0 defeat at Anfield.
Doubles from Divock Origi and Georginio Wijnaldum saw the hosts progress through to the Champions League final for the second time in as many years.
Liverpool went into last night’s game with a seemingly impossible task after Barca’s 3-0 win at the Camp Nou last week.
Lionel Messi turned on the style to score twice along with Luis Suarez’s early deadlock-breaker to give the Catalans one foot in the final.
However, at Anfield it was a completely different story, as Liverpool started the game with ferocious pace, earning the breakthrough after just seven minutes.
The second half started with just the same pace, as Wijnaldum scored twice in rapid succession just 10 minutes after being substituted on.
The fourth goal was the nail in the coffin, as Trent Alexander-Arnold caught Barca’s defenders sleeping in the box and took a quick corner into the area, which Origi tapped in.
And speaking on beIN Sports, Mourinho slammed Ernesto Valverde for not getting his side to be more disciplined with defending the set-piece.
“The fourth goal, if you see that in an under-14 or under-15 match, you’d say the kids are sleeping and have no mentality of football and they have to learn how to play the game,” he said.
“We are talking about the best players in the world, and to concede that goal really shows you a state of mind.
“At half time I said that if Liverpool score a second goal, the atmosphere will be very difficult to cope with, and it was difficult to cope with.
“Especially when Liverpool attacks that end, which is their end. It’s the goal where they love to finish matches and attack in the second half.
“But Barcelona has some of the best players in the world, it’s their natural habitat to play against Real Madrid, Champions League final, Champions League semi-final, every season, it’s something that is so difficult for me to explain and believe that the team is winning 3-0 in the first leg and lose 4-0 in the second leg.
“It’s so difficult for me to believe that in the future something like that won’t happen to my team. But how do I justify that?
“It’s very difficult to justify that. I would like to hear Valverde try to explain that, but I still believe it would be very difficult for him to explain.”