The Swede has been linked with Arsenal and PSG over the past six months. But Varandas says the striker won't be leaving in January.
He told RTP3: "I would say that it is very, very, very difficult, impossible, it doesn't exist in life, now I would say that it is very, very, very difficult.
"He has already scored a lot, he continues to score... it would be very, very, very difficult, very difficult (to leave in January)."
Varandas also discussed claims coach Ruben Amorim could follow departing sporting director Hugo Viana to Manchester City next season.
He added: "When will we lose Rubén Amorim? If it's this year, if it's next year... I've been hearing this question for more than four years and I think this should be a source of pride for all Sporting fans, and also for all the media, who might wonder how one of the best coaches in the world has been at Sporting for five years, it's fantastic.
"The coach always plays in a structure where all the pieces are important, he is the most important piece in that structure, that's how I see it."