In terms of pure cover coming in if Jelavic gets injured, and also maybe for the last ten minutes if needed, he still isn't that great.
What makes you think he is any better than Vellios and Anichebe? He is highly rated because he has pace, strength, and an eye for goal. He's also performed well at youth level and bagged a couple of goals in France. What I think is he is not the sort of signing you should be too excited about in the short term, especially as he will need to settle. Certainly, once he's older he'll surely give you a nice profit, and will improve. Shouldn't be fitting him into your XIs just yet I reckon. Plus his fee will be huge, I believe Arsenal already made an offer of 5 m, and City and Milan are interested. He'll cost a lot and will not give you enough in the short term.
Reckon a winger should be first priority, and a better back-up strike option on the cheap would be better than Niang. Then maybe Niang at the end of the window if needed