The 22-year-old midfielder says that the Spain international's equaliser in their first-leg clash with the Red Devils served as the catalyst for their stirring run to the final
The Basques were tailing to a goal from Wayne Rooney when their centre forward rose to head them level just before the break.
Athletic went on to record a historic 3-2 victory in England before triumphing 5-3 on aggregate courtesy of a 2-1 win at San Mames, and Herrera now admits that they never looked back after Llorrente's equaliser in the first leg.
"It was the moment perhaps we realised we could do something special in this competition; go far," the midfielder told reporters during a press conference on Tuesday.
"It was the moment we made a mark, we showed our ability, our qualities, and that we could beat anyone - it was a launchpad to beat one of the best teams in the world and outplay them."
Herrera added that Marcelo Bielsa's men are now 100 per cent focused on finishing what they started that famous night in Manchester when they tackle Atletico Madrid in the tournament decider in Bucharest on Wednesday night.
"We are determined to end the tournament on a winning note, staying true to our attacking, pressing philosophy and style," the 22-year-old declared.
"We have eliminated and beaten teams older and more experienced than us to get here; we compensate for what we lack with our style, passion and ambition."
The Europa League final gets under way at 20.45CET.