The City manager had described Laporte as the best left-sided central defender in Europe on Friday, a day after he was called into Didier Deschamps' national squad, and was looking forward to his star finally making his international debut next week.
Instead, after Laporte was stretchered off after twisting awkwardly in fouling Adam Webster on 33 minutes, the former Athletic Bilbao defender could be facing a lengthy spell on the sidelines, according to his manager.
"He's in hospital, it doesn't look good," said Guardiola. "I don't know exactly what he has, tonight or tomorrow we will know exactly.
"He is off because he didn't feel good obviously. We will see exactly what he has, I don't know. I'm waiting for the doctor to call me and tell me what he has but he will be out for a while. Hopefully not like Leroy Sane, but we will see."
City have already lost German winger Sane for up to seven months after he suffered a cruciate knee ligament injury in the Community Shield, although his absence has hardly interrupted his team's strong start to the campaign.
However, the loss of Laporte could be more damaging with City having failed to replace Vincent Kompany last summer and with John Stones absent from this contest with an injury of his own.
The Laporte issue was the only black mark on another productive afternoon for Guardiola.
- Clinical De Bruyne -
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City took just 69 seconds to establish their dominance when Kevin De Bruyne finished clinically from 10 yards after Oleksandr Zinchenko had sent away David Silva who pulled the ball back intelligently.
After Laporte's loss, Guardiola had the consolation of seeing City double their lead before half-time after De Bruyne's square ball found Aguero at the end of a flowing passing move.
The Argentinian striker showed great composure in a crowded Brighton area, making space for himself before depositing an unstoppable shot into the roof of the visitors' goal.
A masterful third City goal from Aguero on 54 minutes killed off all hope of a comeback.
Zinchenko's left-wing attack and pass to David Silva saw the Spanish playmaker find Aguero on the edge of the Brighton area and his majestic right-footed curling shot flew into the top corner.
Aguero has now scored six goals in four games this season, matching Edin Dzeko, eight years ago, and Chris Sutton in 1997, for the most Premier League goals scored in an August.
"I think Sergio has scored goals in Argentina when he was six years old, in England with all the managers he has had and with the national team -- he will die scoring goals," said Guardiola.
"He's a special talent and I love that from day one he has tried to do it. We changed many things -- the messages, how we wanted him to play, but from day one I am delighted with his commitment."
City's fourth, after 78 minutes, was hardly needed on the day although it saw Aguero show his selfless side as he found substitute Bernardo Silva in space and the Portuguese international scored from a wide angle with his first touch.