Since City's famous 5-3 win in September, United have not lived up to expectations under Van Gaal.
But Simpson told the Leicester Mercury: "It's a good sign for them that they are not playing great and still winning games.
"It's a new manager, new ideas and new players, so it will take time.
"He is a top-class manager with top-class players, so I'm sure they are more than satisfied with their position and they will be looking for a final push like they normally do."
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