If Arsenal were to loose by 1-0 to Southampton, which could be possible, United need to win by 7-0 to overtake them into 3rd placing.
United should go all out and try after all, United hasn't a big score win this season.
If they were to play the following 4-4-2 formation , they may have a chance to score many:
De Gea/Valdes (De Gea may be rested)
.........................Jones..............Smalling...............Rojo......................Blind
.........................De Maria..........Herrera...............Felliani...................Young
...............................................Falcao.................Van Persie.......................