The 21-year-old is one of the world's most sought-after strikers, with the likes of Chelsea, Manchester City, Manchester United, Real Madrid and Paris Saint-Germain said to be monitoring his situation.
In the summer of 2022, a release clause between £64m (€75m) and £85.5m (€100m) will become active in Haaland's contract, a figure that several of Europe's elite clubs are prepared to pay.
With this in mind, Watzke believes Dortmund may find it difficult to keep hold of their star man, though the club will try their hardest to fend off any potential interest.
"Of course it will be difficult. I am not saying, however, that it is completely out of the question. We'll do everything we can to keep our top players loyal to us in the future," Watzke told German news outlet Welt am Sonntag.
Watzke has also assured Dortmund supporters that the club will "find a new top striker again" if the Norwegian is sold in the near future.
Haaland's strong start to the new campaign continued last weekend, as he netted a brace in Dortmund's 4-2 victory over Union Berlin, taking his goals tally across all competitions to 11 in eight matches.