United haven’t won the Premier League since 2013 and last season finished sixth in the Premier League table.
But Guardiola said he has seen enough during Ten Hag’s first few months in charge to know they are re-establishing themselves as contenders.
“I have the feeling United are coming back, finally United is coming back,” Guardiola told a news conference on Friday.
“I’ve seen it on Thursday [against FC Sheriff] and against Chelsea in the first half. I said I like what I see of United right now. There will be a lot of teams like United fighting.”
United are eight points off the top of the table after 11 games this season and suffered a 6-3 defeat to Manchester City at the Etihad Stadium earlier this month.
“I’m satisfied but there is one team that has been better,” he added. “Today, Arsenal would be the champion.
“We knew it’s not going to be decided before the World Cup. It’s never decided in October, November.
“Of course it’s important to finish as high as possible in the table. We have three games in our hands [before the World Cup] and points to play.
“I have the feeling these three games will say a lot about our, not future, but about what we want to do in the Premier League this season.”