Haaland, 20, enjoyed a sensational breakout season in 2019-20, impressing for both Salzburg and then Borussia Dortmund after a January move.
The Norwegian striker scored 28 goals in 22 matches for Salzburg across all competitions, before adding 16 in 18 outings for BVB.
He was unable to help Dortmund to a first Bundesliga title since 2011-12 as Bayern Munich continued their dominance, but Haaland's swift adaptation provides optimism for the seasons to come – if they can keep hold of him.
Haaland's introduction to international football has seen him score once in three games, that strike coming in Friday's 2-1 defeat to Austria, and the experienced Lagerback believes his potential is almost limitless.
"I've maybe been around too long but I have never seen a guy since maybe Messi or Ronaldo that developed at such a young age," Lagerback said ahead of Monday's Nations League clash with Northern Ireland.
"That's very unique and he has all the possibilities to become a really world-class player – if you look at the offensive third of the pitch and in the box then he's already absolutely top class.
"He perhaps has a little bit more to do in the link play, but being so young, he's already at a really high level.
"With the qualities he already has and his focused personality – if he can stay free from injury then he can be a really top-class player."