I am guessing that it will definitely be big hit for them, but they are a big club and should figure out a way. I know it is always hard to lose a great player and keep the success up, many teams end up losing after they lose a big player, but that is not really impossible to stop, it is something they could do later on and at anytime they could.
This means they could transfer a striker if they lose Jesus for good this season, or if he comes back then they could replace him with some formation change instead of just player change as well to make sure his missing is not a big deal. All in all, they have to endure somehow until the end of the season.
Gabriel Jesus is predicted to be sidelined for three months waiting for his recovery, due to an injury sustained while defending Brazil in Qatar, while Arsenal need striker strength in the second half of the season, because currently they are at the top of the standings and only five points adrift of Manchester City, without the presence of Gabriel Jesus might influence them, moreover there is no suitable replacement figure for him.
Gabriel Jesus is an important player for Arsenal this season, he played a big role in bringing the Gunners to finish in first place in the standings, it is certain that Arsenal must have an option for a striker position, because the quality of the substitutes they currently has not been as good as Gabriel Jesus, so Arteta has to begin to be active in the winter transfer market.