Did someone ask me for a scouting report recently? Anyway, here are some of my thoughts on Diaz.
Overall, he is very much in the mould of what we think of as a "Klopp winger" or wing-forward; I can't think of many players, other than Mane himself, who fit that role on the LW better. He is very quick, technically excellent and gets in goal-scoring positions. He scores a variety of goals and from what I've seen, he is a calm and clean finisher. He enjoys the intensity of the game, tracks back and presses well. He exaggerates contact sometimes, as many players in Portugal do, but nothing too egregious - in other words he doesn't seem like a cunt.
Two things I've picked up recently from watching him in games as opposed to highlights, is that he is pretty good at holding up the ball and that his ability to time a pass is actually superb. When Porto are defending deep, he is usually their most advanced player, much like Salah is for us, and a lot of times has to fight to receive the pass under pressure and use his dribbling skills to protect the ball until reinforcements arrive. There was one moment, I think from a recent game against Benfica, where he did exactly that and slowed the game down, keeping 2 or 3 defenders at bay, waiting and waiting until the precise moment his teammate was about to run into space and found him with the laser-like pass. It's was just superb timing and execution and I can remember several moments like this. Highlight reels tend to emphasize goals and dribbling, but I think passing and awareness of teammates is his underrated strength and in terms of the ability to execute a pass he would be an improvement over Mane. That said, his crossing is nothing special and I don't think Klopp will rely on that part of his toolkit too much.
I'm not sure I can find a perfect example of a player to compare him to, but one name I thought of is Angel de Maria. Diaz seems to have better goal-scoring numbers than Di Maria so far and hopefully that trend will continue, but the similarity is that they are both technically superb players who are also selfless and very team-oriented. You can also perhaps make a comparison to his compatriot Juan Cuadrado in terms of energy and intensity on the wing, although I think Diaz has a lot more attacking nous.
Thinking about how Diaz would fit in this Liverpool team, left-wing seems like by far the most natural position. I already mentioned that Diaz kind of plays a Salah role for his current team, so if we were to imagine him in Mane's place in the front 3, I think he would have to shuttle back and forth a lot more than he does currently, to match Mane's defensive output – that would be something he would need to adjust to. Also, while he does make some outside-in runs playing for Porto, a lot of times he seems simply stationed on the LW and waiting until the ball finds him – in Klopp's system he will have to make a lot more and more varied runs without the ball; I am sure he will enjoy it, but he'll probably need time to build up stamina for that kind of intensity and work for 95 minutes. On the positive side, making more runs will mean that he will be found in space more often, whereas currently he is often surrounded by 2 defenders by the time he gets the ball on the wing, which can be frustrating if that's almost the only situation your teammates can get the ball to you.
I also think it's not completely beyond the realm of possibility to imagine Diaz playing an attacking CM role in Klopp's system – if Ox can do it, I don't see why Diaz couldn't and maybe his ability to thread through a final ball will get more use this way. I don't see him as CF or a RWF in Klopp's system, so I don't think he can be a direct Salah replacement. In summary, Diaz looks like a very natural fit to the way we play and while he will need to make a few adjustments for Klopp's system, all of these potential adjustments seem to dovetail perfectly with the natural tendencies of his game. The one possible concern I have is whether with his seemingly humble personality and upbringing he will be psychologically ready for the kind of iconic status and media attention that being a star player for Liverpool will surely bring. Sometimes that can make players go into their shell; hopefully he'll have enough grounded and low-key characters around to help him feel at home and show the full extent of his talent.
Angel de Maria ? Hmm
I was thinking more of a Colombian Robben.