Slot gets late birthday gift as Isak samples power of Anfield
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Isak, understandably, was some way from his best but showed enough in 58 minutes to give a golden hint to what riches his future may offer.
Liverpool were actually 2-0 ahead before Isak got his first touch in a red shirt after 10 minutes and there were anxious groans around Anfield when he was left banging his hand on the turf in agony after Atletico defender Robin Le Normand's late challenge made painful contact with the top of his foot.
Isak gradually worked his way into the game, but started to look leggy as the first half came to a close.
He came out for the second half, but it was no surprise when he was withdrawn to a generous standing ovation before the hour.
Slot said: "In the 60 minutes he played, he seemed to be quite fit to me.
He could play at this level quite well after one or two weeks of team sessions.
"He can play football and he's a joy to watch.
We play three times a week and that's a bit too much for him still.
"We have two great number nines [Isak and Hugo Ekitike] and we're going to use both of them throughout their career here."
Isak had two shots which brought saves from Atletico keeper Jan Oblak, but the most significant takeaway was his link with another Liverpool new boy, £116m playmaker Florian Wirtz.
The pair dovetailed superbly in the first half to create a chance for Jeremie Frimpong, which he could not convert, then Isak's instinctive flick - with his back to goal and without a look - was played straight into the German's path, bringing roars of approval from Liverpool's fans.
Isak and Wirtz linked up nine times in the game, this small sample size of their work together suggesting they will be a formidable weapon in Liverpool's armoury.
It was a quiet introduction for Isak - but enough to show what he will bring.