Let me say a few thing that could make you think otherwise for Mbuemo.
Mbuemo is an right wing attacker, tested and proven in the EPL, clinical finish if he plays in a club that create opportunities, he has strength and like you said an attacker.. not a striker that will give you almost 20 goals in a season. Manchester United almost went to relegation last season, look at their strengthen no big player will want to come to the club, so I dont know what you expect a world class player. Do you know how much a club will demand if a world class player agrees to move to Manchester United? What they need is exactly current EPL players that can push them up in the table next season, not one they are testing.
You're correct for saying he can push Manchester United to get higher position in the next season because he already played in Premier League for 6 seasons, but he's not for long term project, Manchester United have to find other players, although it's a different problem.
But, honestly they spend too much just to make them safe in Premier League, for me big team like Manchester United would never able to relegated to Championship.