City sold Sterling to Chelsea for a fee in the region of £45million last month, with the England international following fellow forward Gabriel Jesus out of the exit door and to London, with the latter having joined Arsenal along with Oleksandr Zinchenko.
Sterling scored 17 goals in all competitions for Pep Guardiola's team last season, a tally only bettered by Riyad Mahrez (24) and Kevin De Bruyne (19), as they conducted a successful defence of their title, but wanted guaranteed game time that City were unable to offer.
That, in part, was due to the signing of Haaland from Borussia Dortmund, while Julian Alavrez has also arrived to bolster a prolific frontline that features Jack Grealish, Phil Foden and Mahrez.
Yet Sterling's former team-mate and City's record goalscorer Aguero was confused by the decision to sell the 27-year-old to a Premier League rival.
"I don't understand the sale of Sterling," Aguero said.
"There are times they [City] make strange decisions."
Haaland struggled on his competitive City debut, cutting a frustrated figure as Pep Guardiola's team went down 3-1 to Liverpool in the Community Shield on Saturday.
The Norway international only managed three touches in the opening 30 minutes, all of which came in his own half, and he missed the best chance of the game from open play, according to Opta's expected goals model, when he struck the crossbar from six yards out deep in stoppage time.
"He was too used to Germany," Aguero said of the 22-year-old's performance.
"Haaland thought he was alone, then [Virgil] van Dijk arrived and said 'welcome to the Premier League'."
Alvarez did however impress, coming on from the bench to add a different dimension to City's attack and dragging them level when he bundled in from close range following a goalmouth scramble.
Asked what advice he gave to his compatriot when Alvarez signed for City, Aguero quipped: "Julian sent me a message to find out how life is in Manchester. I told him that he was going to be very cold!"
City starts its Premier League campaign away at West Ham on Monday, while Sterling will likely receive his competitive bow for Chelsea at Everton a day earlier.