This is what I was referring to. If a player has a flaw in his game, it's exploitable over any number of games. Why are these four to seven games so different from, say, the 14 games the O6 teams would play against each other in the regular season?
It's quite possible that the O6 period is the exception, as you are correct, teams did specifically game plan for each other's stars in the regular season, and winning the regular season title was much more prestigious than today.
In the modern game, there are quite clearly players whose games don't translate well into the playoffs.
1) Joe Thornton - predictable game - won't shoot the puck. Figure out his passing tendencies, and you only have to worry about his wingers. Compare to Peter Forsberg, who was just as pass-first in the regular season, but became much more of a scorer in the playoffs, and was thus much harder to cover.
2) Keith Tkachuk - became so rapped up in the emotional, physical game that he took himself out of prime scoring positions.
3) Dave Andreychuk (not often mentioned among playoff "chokers" but should be) - one-trick pony - commit defensemen to keep him out of the crease, and he was rendered much less effective.
Canadiens1958 (who I disagree with often - his bias against Europeans and modern players is so extreme as to be comical, for example) has said that Marcel Dionne had very predictable tendencies which could be taken advantage of in the playoffs - which I find plausible, given his consistently mediocre performances.
And in the European area, it's guys like Maltsev, said to not be able to handle the physical play of Canada and Czechoslovakia.
Some GMs look straight at playoff numbers, which is reasonable. I prefer to look at WHY a player didn't perform and to cover up that flaw (as I think I did nicely in my ATD2010 championship team which featured Tkachuk as the 3rd best member of the 2nd line, taking the attention away from him).
As to Cooper, put him with a center who excelled in the playoffs, and he should be fine. As I said, he's a question mark, not a poor performer.