The two sides meet at the Emirates Stadium on Sunday with the Gunners 15 points off league leaders Manchester City.
Despite the gap, Rooney insists that a run of good results could propel Arsene Wenger's side up the table.
"I certainly wouldn't write Arsenal off, they're capable of going on a big run," he told BBC Sport. "You can't write them or Chelsea off.
"It's great for the league that there are so many teams that can win it."
United beat Arsenal 8-2 at Old Trafford earlier this season.