While some may argue it isn't the best approach, or solution, I think we have the players to cover the position. Zlatan is there for now, but even once gone, we have the personnel to fill in the striker role if Lukaku is out. Martial, Rashford, or Sanchez can all be used there and I believe it'll be a rarity to see them on the pitch at the same time, bar a last 15 minute desperation if we need a goal. One of them will always be on the bench at least with Lingard, Mata, or us playing 433 to fill in the other position.
The reason I can see the need of a more 'out-and-out' reserve striker, is that our approach to this kind of cover can be somewhat questionable at full back. Other than Valencia, we haven't really had a settled full back or someone to bring in as a sub that can do the same job as Valencia. We've relied on many utility players such as Blind, Darmian, and Young to fill the void (until Shaw's return to form). The point stands still, if Young & Darmian/Blind are our alternatives to Valencia & Shaw. However, I think the quality of Rashford/Martial/Sanchez is much closer to that of Lukaku - while Blind & Darmian are nowhere near in comparison. They'll do a better job and not weaken us exactly.