We understand the concerns of most Galatasaray fans but we also have to realise that Osimhen is only a loan player who can leave at any time.
Manchester United might be serious about Osimhen and they've made an approach much more likely to sign him and it really depends on Napoli's decision. Galatasaray may need to buy another striker if Osimhen really leaves because Batshuayi is not one of the best strikers ever, especially with Icardi's injury, so with conditions like this, the club will definitely consider buying another striker as a step to maximize the front line to be more solid.
"Leave anytime" isn't how contracts work, if Galatasaray doesn't want him to leave, they do have the right to keep him, there is no January exit clause at all. Its been established and known ever since he joined, so if Galatasaray wants to keep him, then they can. But at the same time, who would want to keep a player who wants to leave? That would be stupid, so its all about Osimhen, but so far he seems like he isn't leaving, well not for United at least, he may leave, but probably for another team, like maybe PSG, which looks like a bigger candidate.
Which team would have such a player in their squad and allow him leave when his contract with them is not expired, even if it were Manchester United or PSG, would they? Victor Osimhen has made so much impact at Galatasaray and letting him leave at this point would be a big lose to them this season, it means they'll have to start searching for another replacement which would be very tough due to Osimhens form, finding a player as good as him won't be easy this January, so they'll try to persuade Osimhen to stay till the season ends. Manchester United and PSG are in desperate need of a striker and they won't back off if an opportunity to sign Osimhen arises. I've seen rumors of United trying to do a swap deal with Osimhen and Marcus Rashford but i don't really believe it since it's not official and Napoli haven't confirmed it yet, moreover Osimhen have not openly shown any interest of leaving Galatasaray yet. Well, i think Osimhen's most likely destination would be PSG cause the most capable of paying for his transfer fee and wages without stress.