Louis van Gaal has challenged Manchester United to continue to improve if it is to win the Premier League or UEFA Champions League during his time at the club.

United moved top of the table with a 3-0 in over Sunderland on Sunday (AEST) but suffered a 2-1 defeat in its group-stage opener to PSV upon its return to Europe's top competition.

Van Gaal says reaching the UEFA Champions League final is a realistic aim for the club but insists he wants his side to improve in order to maximise its silverware hopes.

"In the Champions League you need to win all your home matches, otherwise it'll be very difficult. The important thing is we win," he said of Wolfsburg's visit to Old Trafford. "We've improved that when you compare with the team of last year. We still need time to improve. We have to improve to win the Premier League or a tournament like the Champions League. We're still improving and I see that every week.

"Reaching the final is also an aim. Winning the final is a little bit of luck, I think. It's not only quality, it's also luck.

"Manchester United remembers that fantastically when they won [the final in 1999]. I saw at that time the people of Bayern [Munich] going downstairs and then they scored in the last minute.

"You can say that's quality, but when you've seen the match, you know it's not like that.

"Reaching the final is a fantastic result, but in the media, it's nothing. Second is not the best, and that's all that counts. But as a manager, when you reach the final, you've done fantastically."

Van Gaal was also reluctant to say if he could consider extending his spell with United - though he refused to rule it out.

"Normally I shall leave after next year, but what is normal in our football world, you never know, so I cannot answer," he said. "But I promised my wife to go with her to our paradise so it shall be very hard for me to deny that promise."

Van Gaal - who has ruled Michael Carrick out of the Wolfsburg game due to injury - went on to praise the impact of midfield duo Bastian Schweinsteiger and Morgan Schneiderlin this season.

"You can see that already, he's accepted as a leading player. That's why we have bought him from Bayern Munich," he added of the Germany international.

"I believe in mixtures between youth and older players, and he is an example for the young players. I'm very happy with him.

"We have made progress in the maturity and the balance of the team, that's also why Schweinsteiger is here but also Schneiderlin gives more composure."