The Red Devils have not won the Premier League title since Sir Alex Ferguson's final season in charge in the 2012/2013 campaign.
Under new manager Erik Ten Hag, Manchester United begun this season on a poor note, losing to Brighton and Hove Albion and Brentford in their opening two games but have since strung together six wins in their next nine matches, only losing to Manchester City in that time.
United are also doing well in the Europa League, with a place in the knockout rounds all but secured.
Guardiola has been keeping a close eye on his city rivals and believes they are starting to become a major force again under their new Dutch manager.
“I have a feeling that United is coming," said Guardiola when asked about potential challengers to the Premier League crown.
"Finally United is coming back - I've seen it on Thursday [the 3-0 win over Sheriff Tiraspol], and against Chelsea in the first half [of the 1-1 draw last weekend].
"I said I like what I see of United right now. There will be a lot of teams like United fighting. That's why you have to fight to qualify for the Champions League and fight for the title."
"Newcastle are already there," added Guardiola, whose City side are seeking a third consecutive Premier League title.
"I saw them against Tottenham. The central defenders went man to man against Kane and Son with no problem at all.
"We played them in the third game of the season. Imagine this team playing one game a week? We know how good Eddie the manager is.
"They have new players. They play brave. They are a contender to be there."
