Guardiola has lost four games in a row for the first time in his managerial career, while City remain two points behind Premier League leaders Liverpool, who are playing Aston Villa on Saturday night.
Haaland opened the scoring for City in the first half but Brighton drastically improved after the break and substitute Joao Pedro equalised with 12 minutes remaning.
It was then another substitute, 23-year-old midfielder Matt O’Riley, who came on to score the winner for Brighton on his Premier League debut.
Van Hecke, meanwhile, was involved in a physical confrontation with Haaland inside Brighton’s penalty area in the closing stages of injury time.
Both Van Hecke and Haaland, who pulled the Brighton centre-back to the floor, were booked over the incident.
After the full-time whistle, Guardiola approached Van Hecke on the pitch and was in deep conversation with the Netherlands international.
When asked about his words with Van Hecke after the game, Guardiola replied: ‘That he was strong with Erling and when Erling then stands up, you have to stand up, he cannot go to the grass. Stay strong, no? You pull him all the game then after the fight you stay there.
‘But he played a really good game and congratulations to Brighton.
Guardiola also believes City’s injury list has also been a key factor behind their poor run of form.
‘The point is we don’t have players,’ Guardiola said.
‘I have four central defenders and not one of them is there, all four are injured.
‘Rodri, the best player, is not there. Kevin de Bruyne is a way away from his best. [Jeremy] Doku is injured, Jack [Grealish] is injured.
‘So we can do one game, we can do it, but to be consistent is not. In the second half I can use Manuel [Akanji], or Nathan [Ake], or Ruben [Dias] and after that in the last moments I need to change the game I put Doku or another player.
‘But it looks like, please don’t misunderstand me, it looks like that I’m complaining, it’s not, we played really good from my point of view. But to play three [consistently]. What is [different] to the previous seasons? We had the squad, now we don’t have it.
‘This is the main difference but we have to survive with that, to live with that.’
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