Dries Mertens will play as a left winger, Hakim Ziyech plays as a right winger. I think Galatasaray coach will use this duo players for the wingers. But it will be quite confusing if one of this players got injured. There is no more option for a left winger if Mertens injured. Well, I can understand that Galatasaray has too many right wingers, they have no pure left winger.
However, it is good that Galatasaray signs Osimhen. It is a food addition player for Galatasaray. With 4 strikers in the squad, I think Galatasaray coach may use an option to play with 2 strikers.
With Mertens and Ziyech at left and right wings, Osimhen in the center will receive good key passes, cross basses and assists from his team mates in this season. Mertens used to be a main scorer for Napoli some seasons ago and even he is older, he has great skills to score goals. Ziyech is younger than Mertens and he will support Osimhen better as with less injuries, more appearances, he will have more chance to play with Osimhen in Galatasaray.
Mertens with his old age, has bigger risk from injuries and might miss many matches for Galatasaray in this season. It limits his chance to play together with Ziyech and Osimhen and possible appearances for Gatalatasaray as well.