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.
Osimhen with this transfer to Galatasaray actually made a big mistake and it starts a downside of his career. He will need to find a better club after the end of this season, possible destination can be La Liga or Premier League if he wants to play at highest level.
I do not agree with this because I have feeling he has done better decision because sitting on bench is not ideal for any star like him with Napoli is already having no desire to have Osimhen into their playing eleven specially as now they have Lukaku so Osimhen were had only 2 options go to Saudi League or have something other option so he used to go Galatasaray which is also having commitments in UEFA Europa Cup, so things will be favorable for him and his performance in this season.
If we have good performance from him in this season then surely we can expect good club for him in coming season as now release price is also having good decline things are not bad at all Turkish clubs are having good hype in last few months, so things can bring good change for his career in UEFA.