To be fair,
if the op is completely de-conditioned strength-wise, getting the deadlift stronger would undoubtedly help (
and some coaches would claim that it would help a lot). Or, to put it another way, all else being equal, the guy deadlifting 495 is
probably a faster sprinter than the guy deadlifting 135. Obviously, the same logic applies to the olympic lifts (300lb clean faster than 100lb clean), but again, if he is new to strength training, the basics (squat & deadlift) must be learned first. Due to the complexities of the o-lifts, the squat & deadlift would probably give much faster returns to a newbie trainee so that he could see improvements in the current season, rather than a year later when his o-lift technique is up to snuff.
Having said all that, doing lots of running - in the forms of intervals and occasional all out sprints - plus general conditioning (to last an entire soccer game) is probably going to have the most immediate impact. Only once the running training is showing diminishing returns would I start thinking about strength training and only when that the squat & deadlift are very efficient would I worry about o-lifts. All of this, of course, assumes soccer performance & speed are the primary goal.