Arne Slot was left surprised by Alexander Isak's fitness after he made his Liverpool debut in their late 3-2 win against Atletico Madrid on Wednesday, but the Reds boss wouldn't confirm whether the striker would start against Everton in the Merseyside derby on Saturday.
The £125m British record signing was handed his first start for Liverpool as the Premier League champions let slip a two-goal lead after efforts from Andy Robertson and Mo Salah were cancelled out by Marcos Llorente's double, before Virgil van Dijk headed home in the second minute of added time.
In a disrupted pre-season, Isak did not play a part in Newcastle's summer preparations as he sought a move to Anfield.
And the 25-year-old's only appearance of the season before Wednesday was an 18-minute cameo for Sweden against Kosovo.
Asked if Isak's display had surprised him Slot, who celebrated victory on his 47th birthday, said: "Particularly because I think in the 60 minutes he played, he seemed to be quite fit to me - so it wasn't as if, after 10 minutes, he was already very tired.
"He could just play at this level quite well so that was the most positive thing - and that he can play football, and that he's a joy to watch.
"That is not a surprise to me, but he was maybe fitter than I expected.
"I was positively surprised by how fit he was. I was not surprised by the quality but it is always nice to see when a player starts how he started. It was a good start but only 60 minutes.
Slot went on to say he was unsure whether Isak will start again on Saturday at Anfield against Merseyside rivals Everton but added: "You can tell (Everton boss) David (Moyes) he is not going to play 90 minutes.
"I'm really happy he could start today. That's why we didn't play him against Burnley. He was good today and when Hugo (Ekitike) came in, he was a big factor also.
"We have two great No 9s and we're going to use them, both of them, throughout the whole period they're here."
Slot lauds Liverpool mentality
Liverpool have made a habit of scoring late winners this season, most recently beating Burnley 1-0 in the Premier League thanks to Salah's 95th-minute penalty and scoring on or after the 83rd minute in each of their games.
"If you want to beat a team like Atletico, which is known and has an unbelievable mentality, you have to beat them with their own weapon as well and that is mentality," Slot told TNT Sports.
"I think that's what we showed again today, that we can beat them also by mentality. But I think we should have made it easier for ourselves."
Robertson: We need to make it easier
Meanwhile, Robertson, credited with the opener after Salah's fourth-minute free-kick deflected in off the defender, also felt his side's win should have been more comfortable.
He told TNT Sports: "I think we need to maybe get back to winning a bit simpler and a bit easier.
"Obviously it's a great thing to have been able to keep going right to the end but when you're 2-0 up and you're playing so well in the first half, it should have been a more comfortable night."
On Isak's long-awaited debut, Robertson added: "I thought he was excellent. He's not really had a pre season, not had a lot of games since last season, but I thought, especially the first half, he was excellent.
"Naturally he got tired and then it's amazing to have a player like Hugo to come on and both of them have got such quality."
Carragher: Van Dijk to go down as Liverpool great
Jamie Carragher says Van Dijk should be remembered as one of Liverpool's greatest ever players - not just one of their greatest centre-backs after he scored the Reds' stoppage-time winner against Atletico.
"I don't like describing Van Dijk as the best centre back. At Liverpool, a lot of people get called legends because they've won the league or the Champions League," Carragher told CBS.
"There are lots of them. But if you want to differentiate, and I call them giants of the club like Gerrard, Dalglish, Hansen, Barnes, Souness and Salah, the top seven or eight players at Liverpool, he is right in the middle of that.
"He's not at the bottom of the list I can assure you. He will go down as one of the greatest players to ever play for Liverpool. Not one of the greatest centre backs, one of the greatest players."
Simeone: Fan insulted me - I'm human
Atletico boss Diego Simeone was shown a red card after a heated exchange with a supporter behind his side's dugout.
Stewards had to intervene in the flashpoint before Simeone was sent off by referee Maurizio Mariani.
"They were shouting insults through the whole game from behind the bench and I cannot say anything as I am a coach," the Argentinian said.
"Obviously, my reaction is not justifiable; I insulted him. But it was 90 minutes of being insulted the whole time then you turn around, when your opponent has scored, and they are still insulting you. It is not that easy."
Simeone said he hoped the Anfield club would take action.
"I hope that a team like Liverpool can improve that part," he added.
"When they identify the person who did this, it should have its consequences.
"But the person who needs to stay calm, and take the insults, is me. In my place you just have to take it."
Simeone refused to be drawn on the exact nature of the insults, declining the opportunity to clarify his words post-match.
"I am not really going to get into the exact nature of the insults. I don't want to get into that discussion," he said.
"I know what went on behind the manager's bench. I cannot solve society's problems in one press conference. I have to live with it because it happens all the time."
UEFA will await the referee's report before deciding on any possible sanctions following Simeone's red card.
