The 25-year-old Norwegian became the quickest player in Champions League history to hit 50 goals in what was his 49th appearance in Europe's elite competition when he cleverly headed home Phil Foden's perfectly-placed flick in the 56th minute.
He smashed the record held by former Manchester United and Real Madrid striker Ruud van Nistelrooy, who reached 50 goals in 62 games.
"The number speaks for itself," Guardiola said. "He's alongside in terms of goals with Van Nistelrooy, (Robert) Lewandowski, but especially with the two monsters Cristiano (Ronaldo) and (Lionel) Messi over 15, 20 years. Erling is there. In terms of goals, it's unbelievable."
Ronaldo is the all-time Champions League top scorer with 140 goals. Asked if Haaland - given his numbers at such a young age - could one day top that mark, Guardiola did not hesitate.
"If (he keeps up) that rhythm? Yeah," the Spaniard said. "He can play I don't know how many years, 10, 12 years, I would say if he maintains this progression, absolutely."
Guardiola was equally effusive about Foden, who scored in City's 3-0 thrashing of Manchester United on Sunday in his first start of the season and was excellent again on Thursday.
Foden was named PFA Player of the Season for the 2023-24 campaign but his form vanished last term and the goals dried up.
"You cannot imagine how we missed Phil," Guardiola said. "Last two games have been the Phil that we know. He played close to Erling in the year we won our fourth (consecutive league title in 2024). Phil was the best player in the Premier League.
"It's the sense of danger close to the box, and he could score one or two goals again," Guardiola said. "So yeah, we are so happy, because I know if he's happy and his life is fine, and the rest, his natural, incredible talent comes along."
Guardiola said there was never pressure on Foden to turn around his form.
"Everybody in our life, we need to be loved, but he is special," the manager said. "There are people that are more sensitive, and he's a guy who needs to feel embraced, in all the aspects, with mates, with me, with the club. When that's happened, Phil responds like the player that he is."