This is trotted out quite frequently as a convenient and pithy rebuttal of a person's argument. However, I think this is a specious argument that says more about the judge than the player because the evaluation of a player is never going to have a data set full enough to provide 100% surety and 0% risk in a player's future performance.
That said, my contention is that you can absolutely evaluate a player on 90 minutes of football depending on what assessments you are looking for, provided they get through enough of the ball in that 90, and the level of competition is of a suitable standard. If watching a player for a season will give you 90% confidence in a players future performance (pulling the trigger on a transfer) I would like to believe that 90 minutes should give you about 70% confidence ie should I come back to see this guy or not.
There is a point where the evaluation of a player changes from 'are they good enough' to 'what is wrong with them'. And this is the part of it that the "No to 90 minutes" folks don't understand. It takes significantly less time to know if a player is good enough than most people believe. The rest of the time spent trying to find a player's warts and the novel situations that bring them out.
A for instance for that last part. When Tripps was being recruited everyone and their brother trotted out his fecking assist stats but few mentioned the potential pitfalls. Playing for Burnley over a 38 game season likely never put him in a situation of defending high up the pitch or running back to his goal ie novel situations. He had no field to defend at Burnley and was free to flight in crosses because that's Burnley's move.
Evaluation Tripps for a single 90 minutes would have given you the same impression as over a 38 game season, this guy is good enough (if you're Poch and many on these boards). But evaluating Tripps over any 1 game for Spurs this season would evince his warts and give you pause (unless you're Napoli or some fecker whose idea of a good time is to coat himself in soft-ripened cheese and fruit jellies while pondering the making of his next outfit and tending to his sphinx moths).
So, one 90 can be enough provided the competition is suitably high, the player has enough ball, and you know what you are looking for. Should players be looked at only for 90? Obviously not. But I believe also believe that a fairly confident assessment can be made in about 10 touches but I won't make that argument here as I fear for the local population around
bus-conductor
's place.