When injury denied Jürgen Klopp the services of Joe Gomez, Ragnar Klavan and Joel Matip ahead of the first leg against Manchester City on Wednesday night, the Reds boss sought defensive back-up in the form of the U23s centre-half.
Masterson had previously made the bench for the senior team when Klopp selected a youthful travelling group for the FA Cup tie at Exeter City in January 2016.
But this was another level for the Republic of Ireland youth international, who handled the opportunity with maturity beyond his 19 years.
“It was an unbelievable experience from start to finish,” he told Liverpoolfc.