When I first heard that Denard said this, I was pretty surprised. Yes Usain Bolt is not a great starter, but even over the short 40 yard distance, I would guess that he is faster or at least as fast as the fastest football players. Lets look at a breakdown of Bolts 9.58 second 100m run at the 2009 world championships, to show why I think this.
As you can see above Usain Bolt takes 4.64 seconds to reach 40 meters. However, notice that reaction time (Column RT), this means the clock starts 0.146 seconds before bolt even twitches his first muscle. Combine 40 times do not start until the player is visibly moving to the human eye. So, as a first step in figuring out Bolt's combine time we must subtract the reaction time from his 40m split, this gives Bolt a 4.494 combine time if 40 yards was the same as 40 meters. But of course there are only 36.576 meters in a 40 yard dash. Therefore, we must multiply 4.494*(36.576/40) = 4.11 to get the true 40 yard time. So 4.11 seconds would be his 40 yard time using electronic timing methods. However, we are not done yet. Despite living in the 21st century the NFL combine still goes by a hand timed start. (they do perform electronic timing, but those times are never released for public consumption). On average a hand timed start will take off 0.21 - 0.24 seconds in a 40 yard dash. This leaves us with a 40y track time of well under 4 seconds for Usain bolt, 3.87 - 3.90.*
Now we must acknowledge the fact that running on turf is going to be a bit slower than running on a synthetic track with starting blocks. How much slower? Thats something no one has quantified, but I think its less than 0.3 seconds (If only Reggie Bush ran a combine 40, we'd at least have one data point on that. How much slower than 4.33 do you think bush is in a 40y). I'm sure if Denard trained for years exclusively for sprinting he could probably beat Usain Bolt in a 40 but right now I just don't think he can. What do you think?
*An article on combine timing can be found here.