Meulensteen was reacting to Monday night's 4-0 defeat at Crystal Palace.
He told BBC Sport: "From my own experience, for any manager that is given a new squad and new players then it will take about 18 months for that to bed in, to get everyone to buy in to your way and what you want.
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"Erik ten Hag is beyond that 18 months and after a reasonable season last year, this should be the season that the squad kicks on. But the problem is they have regressed.
"Inside 18 months, you've got the players you want, you know what you want from them and they know the expectation, and then you get the consistency in results and performances.
"That has not happened, and that is worrying when these were seen to be the group of players that were going to shape the future of the club."