The Red Devils suffered a 3-2 defeat to Andre Villas-Boas's side at Old Trafford on Saturday.
"We have to be positive," he told MUTV. "It's not a bad start to the season.
"We won at Anfield [against Liverpool], which we didn't do last season, even if they played much better than us. We still have a lot to do, but we have to forget that first half against Spurs.
"I hope it was just an accident and we can get back the winning mentality in the Champions League [against Cluj] and against Newcastle."
The defeat has pushed United to third in the league table with 12 points from six matches, four points behind leaders Chelsea.