A significant injury to the league's top scorer would be a huge hit to City's bid to retain the title with the team winless in their last four league games.
"A bone stress reaction in his feet," Guardiola told Sky Sports ahead of Sunday's kick-off when asked why Haaland was not in the squad. "After the last game against Aston Villa he was like this and so he cannot play.
Asked about the severity, Guardiola said: "I don't know. Week by week, day by day, we'll see what happens."
The 23-year-old has been struggling with a foot injury, withdrawing from the Norway squad during the recent international break to return to Manchester for treatment.
A stress reaction, which includes swelling and bruising in the bone, can progress to a more serious stress fracture.
Haaland has 14 league goals so far this term and last season broke the record for goals in his debut campaign, scoring 36 in 35 appearances.
