Manager Jurgen Klopp revealed Philippe Coutinho had lost three kilos in weight in the last three days before the sick star dragged himself off the bench to help Liverpool beat Stoke City 2-1.
With Klopp's side trailing to Jonathan Walters' 44th-minute header, goals from half-time substitutes Coutinho and Roberto Firmino turned the game on its head midway through the second half on Saturday.
Liverpool have picked up 16 points out of the last 18 available and are six games unbeaten, cementing third place on a day when top-four rivals Tottenham Hotspur and Manchester City also won.
But, aware Coutinho was struggling with diarrhoea and Firmino had been fatigued, Reds boss Klopp admitted it was a big decision to bring on the Brazilians at the break as he feared they would only last 30 minutes.
"At half-time I had a difficult decision to make because it wasn't clear that Philippe or Roberto could play 45 minutes," Klopp said.
"Phil lost 3.1 kilos in the last three days which isn't good for a professional footballer.
"He came to me this morning at the hotel and said 'I'm fine' but we knew he would have low energy levels. We thought he might be good enough for 30 minutes.
"Roberto said 'I'm really done' after two games for Brazil. The last game after injury he played against Man City, he flew immediately to Brazil, played there, came back to play straight away in the derby, so 90 minutes was going to cause him real problems.
"The way we understood it, the possibility was pretty high that we'd have had to change after 30 minutes.
"We have a lot of numbers from sports science and this was the moment when each alarm clock was ringing for both."
- Save of the year -
Klopp showered praise on goalkeeper Simon Mignolet after he superbly denied Charlie Adam and Saido Berahino in the second half.
"We needed to fight and Simon Mignolet made two outstanding saves -- the second one was one of the best I've ever seen, it was save of the day, the month and the year," he said.
"When a goalkeeper makes saves like that, it's like scoring a goal."
Klopp also said youngsters Ben Woodburn and Trent Alexander-Arnold, who were making their first and second Premier League starts respectively, showed a disappointingly selfish attitude after making way for Coutinho and Firmino.
"Those players didn't celebrate immediately with the boys," he said.
"They don't have to react like that; they can be disappointed but we will help them more than we help anybody else.
"It's not about them. It wasn't as if we didn't use them -- we involved them in the game."
Stoke boss Mark Hughes believed his side were hard done by after a fourth straight defeat.
"It's a difficult result to take. We were by far the more dominant side during the first 45 minutes and were full value for the one-goal lead at the break," he said.
"They created three clear-cut chances and scored two of them and we created more than that and only scored one -- it's just the way things are going for us at the moment."