The England winger picked up a thigh strain on international duty and has failed to find the net since returning from the bench against Norwich City.
Walcott has insisted that his personal ambition of reaching 20 goals for the campaign must serve as a secondary goal to ensuring Arsenal secure a top-four finish and Champions League qualification.
He told the club's official matchday programme: "Getting injured on the international break was frustrating, and the one thing I've been a bit disappointed with since then is the goals element. I haven't scored for a while now.
"But there are only four games left now and it's not the time to think about myself. I set a 20-goal target for the season and only need two more to reach it, but the biggest goal for me is to help Arsenal finish third or fourth.
"That said, I'd be perfectly happy with a goal against United to kick off a great end to the season."
The 24-year-old has found the net 18 times this season in all competitions, but is without a goal for the Gunners since January.