We should start with striker as we have 2, one in **** form and the other plays on the wing half the time. As for wing players, we have Depay and Mata (I agree, poor form), but we also have Valencia, Young, Lingaard, and I can mention Martial here as well. That is why we need the striker most.
If we were to sign a winger and had Martial and Mane there, what happens if one gets injured? We have a wealth of options to replace them with. Whereas if one of Rooney or Martial gets injured where is the replacement?
Again, I am not saying sign one player and be done with it, I am saying put effort, money and time into finding the best striker we can get and not just a panic buy striker. We need to invest in this position now. We may have loads of money but certainly not enough to sign both Mane and a great striker. Seeing as buying mediocre strikers will only worsen the situation in the long haul.
Our wing options are poor except Martial, so it's very important to sign one who is direct and beat his man.
If Rooney is injured, Martial can lead the line. If Martial is injured then we are ****** anyways even if we sign winger or striker.
Besides all these there isn't a very good striker available in Jan, whereas by the rumors Mane is. It would be great if we can sign Lukaku but that's looks unrealistic in Jan.