The 18-year-old striker has scored four goals for his club Falkenbergs this season.
And that form has led to speculation over a possible move to Old Trafford.
But while Karlsson admits the link is 'fun', his focus is on playing well.
He told Swedish newspaper Aftonbladet: "Of course it’s fun, but I cannot do anything more than just go out, do my best and perform.
"If they check and they think I’m good, that’s just a plus."
It is claimed in the report that Karlsson has a contract with Falkenbergs until 2018 and director of __football Hakan Nilsson refused to dismiss the suggestions of interest from United.
He added: "It’s a possibility they [United] have expressed an interest. Just like a number of other major clubs."