I haven't overestimated anything. All I've said it has a negative effect and that's true. To say otherwise is really foolish.
Many of the effects are short-term, which for a NFL player using during the off-season would have a negative impact on their training for the upcoming season. Let's not forget that we're talking about a sport where the difference between a starter and reserve player, or a reserve player and someone that's cut in training camp, is minimal. It absolutely has a negative impact.
ESPN.com - SPECIAL - Marijuana
What are the effects of marijuana on performance?
•Impairs skills requiring eye-hand coordination and a fast reaction time
•Reduces motor coordination, tracking ability and perceptual accuracy
•Impairs concentration, and time appears to move more slowly
•Skill impairment may last up to 24 to 36 hours after usage
•Reduces maximal exercise capacity resulting in increased fatiguability
•Marijuana has no performance-enhancing potential
Because marijuana is stored in the body fat, its effects may be long-lasting. "It has been shown that performance skills can be impaired for as long as 24 hours after marijuana usage," says Wadler, "which casts doubt on the commonly held belief that the social use of marijuana the evening prior to an athletic event will not affect performance."