I'm not sure we need to go out and buy another striker as such. I remember seeing a post mentioning how Lukaku rarely misses games, he seems to always be a reliable part of the starting eleven. Now, obviously he's not immune to injuries but, that's a good sign.
We also have Martial and Rashford as others have said, Rashford has a habit of missing quite a few chances, I really hope it doesn't continue into the season, but we saw one in particular against West Ham and one against Real Madrid. Martial is cold in front of goal, nerveless, but I think he's better from out wide. I'm often torn between the two and which player should play.
As for Zlatan, I'm indifferent as to whether he comes back in January, it would be a boost to the team if we're in a position of challenging on a couple of fronts, but I wouldn't cry any rivers if he went elsewhere.
I'd also like to add that the last game we managed two goals from the striker, one from a CM and another from a winger. That share of goals looks healthy, hopefully the other players start chipping in and not criminally missing chances.