Jurgen Klopp admitted Liverpool's FA Cup third-round tie was a step into the unknown after his side emerged with a 4-1 win over Aston Villa's youngsters.
Sadio Mane's double and goals from Georginio Wijnaldum and Mo Salah sent the Reds through to the fourth round.
But the Premier League champions were stunned when Louie Barry equalised for the coronavirus-hit hosts before the break.
Villa called on their youngsters after nine players and five staff caught Covid-19. None of the senior squad were involved and the game was in doubt until Friday morning after the club shut their training ground on Thursday.
Manager Dean Smith was also absent and Klopp, who started Salah, Mane, Wijnaldum, Fabinho, Jordan Henderson and James Milner, conceded he did not know how to prepare against Villa.
He said: "It was a tricky one. I have never had this kind of challenge in my life when you have no idea who you are playing. All our preparations for the game went in the bin and we had to start again.
"We scored an early goal and then didn't play quickly enough and didn't move fast enough. These are football problems and we solved them with football in the second half.
"Academy players are good players. Last year we played here with our kids and they gave Villa a proper game as well. If you don't play well you have problems, but in the second half we played like we should have done from the beginning.
"We trust the authorities. Our test results came back yesterday and everyone was negative. They did what they thought was right with their under-23s and everything is fine."
It looked like Liverpool would steamroller the hosts, made up of players from the under-18s and under-23s, when Mane headed in Curtis Jones' cross after four minutes, but they failed to build on the lead.
Villa's 19-year-old goalkeeper Akos Onodi denied Fabinho and Mane before Barry stunned the visitors.
Callum Rowe's fine pass sent him clear and he held off Rhys Williams to finish past the exposed Caoimhin Kelleher five minutes before the break.
Liverpool, though, pressed in the second half and scored three goals in four minutes and 20 seconds.
Wijnaldum found the bottom corner from the edge of the area after an hour, before Mane looped in a header for his second two minutes later.
Salah, who earlier had a goal disallowed for kicking the ball out of Onodi's hands, then sealed victory when he turned and fired in low after 65 minutes.
Thiago Alcantara hit the bar with five minutes left, but Villa Under-23s coach Mark Delaney praised his players, with many having been driven to the game by their parents as they are yet to pass their driving tests.
"I've said proud quite a few times. As a club, the badge I wear and the coach I am I'm immensely proud with what the players have produced against one of the best teams in Europe," said the former Villa defender.
"It's an unprecedented situation and I spoke to them and they have to take real pride, real courage and real understanding (from the game).
"There is a long way to go, but if you produce an attitude and work ethic like that and bring the quality with it you can go wherever you want.
"These lads have had two days of training and four weeks off since the last time they played. Liverpool showed us a lot of respect for the substitutions they made early on after the situation we put them in."