Forestieri was the subject of a bid of a reported £7million from Championship rivals Derby County earlier this week which Wednesday rejected.
Concerns have been raised that the forward’s mind may not fully be on the job when the Owls travel to Norwich City this afternoon.
But talismanic defender Lees – himself the subject of transfer talk this summer – dismissed those concerns outright.
“All I can say is that so far, it hasn’t affected him,” Lees said. “He’s been in great form and hopefully that can continue.
“We’re all right behind him as well so nothing really changes.
“There’s always speculation whether it’s Nando or someone else. We don’t really take any notice of it.”
Lees says Wednesday simply have to hold onto Forestieri and star men of his ilk if they are to achieve their goal of a return to the Premier League.
He said: “Every club, if they have got any ambition of wanting to do anything, wants to keep their best players – Fernando definitely falls into that category.
“He knows what everyone in the dressing room thinks of him and how highly we regard him.
“If you are asking me, I think he really enjoys playing here but it is probably nothing to do with that. It is obviously down to him and the club.
“He won’t let it be a distraction as we’ve seen how well he has done in pre-season. Hopefully he can carry on doing that.
“He’s vital for us. Players who change games at this level are pretty rare so I think we need to have him firing for us.”