"It is realistic to expect us to win the Premier League and the Champions League (before my contract expires). Because I did it everywhere," Van Gaal told reporters ahead of Wednesday's Champions League clash against VfL Wolfsburg at Old Trafford.
United sit on top of the Premier League table after seven games, but have made a poor start to their Champions League campaign, having lost 2-1 to PSV Eindhoven in their first match.
"We have improved when we have to compare us with the team of last year, but we need still time," the former Netherlands manager said.
"We have to improve to win the Premier League or a tournament like the Champions League."
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Van Gaal said he was still planning to leave after his contract expires in 2017, reluctant to break a promise to his wife that he would retire as planned.
"Normally I shall leave after next year but what is normal in our football world? You never know so I cannot answer that question," Van Gaal said.
"But I promised my wife to go with her to our paradise (villa in Portugal) so it shall be very hard for me to deny that promise.
"Next year I am 65 and say I stay a year longer, it shall be very hard to my wife," he added.
(Reporting by Shravanth Vijayakumar in Bengaluru; editing by Sudipto Ganguly)
