With three of United's next four games against league leaders Chelsea, champions Manchester City and Arsenal, Van Gaal is about to find out whether his £150million summer makeover is built on rocks or sand.
Fellaini said: "We know we have a problem with the balance between defence and attack, but we train all day to try and make it better.
"We offered too much space in defence (against West Brom) but next week is a big game and we have to learn to focus for the whole game. I am determined to get things right - that is why we play football.
"I don't think it was a good result for us (on Monday) because the next two games will be difficult."
For the big stories and the banter follow us on Twitter: @tribalfootball