The Belgian midfielder, who came off the bench to help United claim a 2-2 draw with West Bromwich Albion on Monday night, claims that Louis van Gaal has been attempting to improve his side's ability to change approach depending on the match situation.
"We know we have a problem with the balance between defence and attack, but we train all day to try and make it better," Fellaini told reporters.
"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."
Van Gaal's side host Chelsea at Old Trafford on Sunday.