Seems to be a lack of distinction between 'pace' and 'acceleration' here and that one is far more important for a striker than another.
Acceleration is by far and away more important than pace because it enables a striker to get away from his marker and get his shots off before defenders can react. It also enables a striker to stand his man up, from an upright, almost walking or even an outright dead stop position and then leave him for dead.
With Greenwood's shooting ability and the time it takes him to draw his foot back and connect with the ball with unerring accuracy, it should be clear how invaluable short-burst acceleration is. If he is to play further away from goal, say as a deeper forward or a wide attacker, then sure, pace and top-end speed become more important to factor in but for the striking position of a typical #9: a Romario, Aguero, RVN, Lewadowski etc. creating that yard of initial space is paramount.
Greenwood isn't explosive and he probably won't be top-end in the game in that regard, but he is fast enough off the mark to execute the ideas he visualises and has done so without issue or hindrance to this point in his career as both a youth and now a full professional.
His 'thing' is intelligence, movement and opportunism - he reads not only the play but his markers well and times his actions to match theirs, i.e. he'll take his shot just as a defender plants his feet and sets himself to try and close down, or he'll go left as his man assumes right and in that window, he's already done what he wanted to no bother.
I would hold off judgement on his physical strength, power and pace until he is a couple of years down the line in his development, personally. Growth spurts, filling out, gaining mass and so forth can all contribute to a different physical proposition than a kid just coming through. Of course, you have your Ronaldo's (both), Rooney's, Giggs', Messi's and so forth who you can see the template of from the outset, but there are players whose physical presence is not realised straight away, and besides which, it should be considered that at just-turned 18, it's generally only freaks and prodigies we see at this level of football so we're witnessing a youngsters' growth where for a lot of those in his age range, they're quietly going about their business of developing away from the masses and then start to emerge in the men's game at around 20ish or so.
I should think Greenwood will not look like he does now at 20 years of age. And I wouldn't be in the least surprised if his centre of gravity and core strength become much stronger than it is now and leads to a quicker player off the mark than he is now. He's never going to be an outright pace merchant, but he really doesn't need to be to be a world class prospect. In other words, it's a not case of 'he'd be so much better if not for his lack of <insert>' as you might say with others who emerge but clearly lack a key physical attribute or two for their position on the pitch.