Granqvist, 33, impressed alongside Victor Lindelof in the centre of Sweden's defence in the summer as they reached the last eight of the World Cup.
He appeared on Swedish podcast Road to the Top earlier this week and was asked about potential interest from United.
"Should a club like Manchester United show an interest in January, it will be very hard to say no," Granqvist said.
"It's great to even be mentioned with a big club like Manchester United, it's incredibly flattering. It's a sign that I did a very good World Cup.
"I was incredibly pleased with my performances in 2018. We'll see what happens. I do not think it's so great, but there's certainly something behind it. Then we'll see how much interest is in January."