Two second-half goals from the Norwegian secured a vital win for City in Portugal as Enzo Maresca started his first UCL campaign at the club with victory.
Summer signings Enzo Fernandez and Ayyoub Bouaddi started at the base of City's midfield after joining the club for a combined £200M+ this summer, and it was the 18-year-old who caught the eye at the Estádio do Dragão.
The former Lille playmaker has only made appearances off the bench so far under Maresca - who praised his calmness and maturity after some nervous moments - and Haaland believes he could be a difference maker this season.
"He did a really good game. The last couple of weeks in training, he's been amazing. I think we have got a gem there, to be honest."
Bouaddi completed 54 of his 55 passes in Porto, while also topping the charts across players from both sides for the most dribbles (4), most times possession won (7) and he contested more duels (13).