The Norwegian has struck 23 times in 13 games for club and country during a blistering start to the season.
Haaland's double took City top of the Premier League, for a few hours at least prior to Arsenal's visit to Fulham.
However, Guardiola is concerned by his side's over-reliance on their number nine.
"Really pleased but we cannot rely on just him, we need other players," said Guardiola.
"Wingers, attacking midfielders. They have to make a step-up and score goals."
Haaland accounts for 11 of the 17 league goals scored by City this season.
"We play to provide Erling with quality and passing but other players have to step up.
"The chances were clear, and they have to score," said Guardiola.
"At this level, they have to demand themselves. Chances from Savinho and Jeremy (Doku), and Tijjani (Reijnders) - they have to score.
"They have to do it otherwise, we can't do what we want to do."
City finished a distant third to Liverpool and Arsenal last season as their run of four consecutive titles came to a crashing halt in a trophyless season.
After two defeats in their opening three Premier League matches, Guardiola's men are now unbeaten in eight games in all competitions and look like title contenders once more thanks to Haaland's form.
"Erling lives for it," added Guardiola.
"Our delivery of passes has been better, and Erling is our key man right now."