Rooney equalled the scoring record for England at the weekend, but he remains 17 away from securing the same achievement for Manchester United. He's been in decline ever since his first round of hair plugs in 2011, and that was exacerbated by his top-up treatment in 2013. The midfield experiment of 2014/15 has since ended, and there are no other serious contenders for his position. If he does not find something like competent form, United's season is very probably already over. It's a shame for Rooney and United, but the current situation cannot go on for much longer - the only option is that he is dropped or plays himself back into form.
If he's dropped, then it is probably Martial who takes over from Rooney in attack. He's not weak, at all, so should be able to lead the line against most teams. But he is inexperienced, and not yet lethal in front of goal. He has pace and impressive technical ability, so he already offers more - on paper, if not practice - than Rooney currently does. It would be a brave player and manager who starts him regularly this season and places much responsibility upon him. Expect him to play mostly from the wings or as a substitute.
He was almost sent on loan to West Brom at the end of the transfer window, hinting that Louis van Gaal had someone else - Marco Reus or Antoine Griezmann perhaps - as a replacement. Instead, nobody else arrived on deadline day, and he was retained for the first team squad. It would be no surprise, then, if Marouane Fellaini actually plays as United's senior second-choice striker.