Bellingham's strong start to the season has seen him register 12 goal involvements in all competitions, earning him a place in England's FIFA World Cup and garnering interest from a number of top clubs.
Liverpool, Real Madrid and Manchester City are all rumoured to be interested in the midfielder, who signed for Dortmund from Birmingham City as a 17-year-old in 2020 for an initial fee of £25million.
If Bellingham does leave, Dortmund reportedly want a fee of €100m, though that number could end up reaching €150m should a bidding war play out for his services.
While Bellingham is yet to publicly state any desire to leave Dortmund, Watzke knows will be difficult to keep hold of him if that changes and the top teams come calling.
"We will hold talks now, as soon as [the World Cup in] Qatar is over," Watzke said. "Then we'll have a fundamental talk about what he would like.
"He just needs to tell us if he'd like to stay or if he'd like to go. In both cases we'll talk about it very pleasantly and reasonably together.
"In general we can imagine that he'd stay with us, but we can't behave as if this topic wasn't on the table.
"If he says 'I'd like to do something else,' then the really big [clubs] in Europe will be there and we can't afford to fight financially."