I think it's quite fair to compare Coutinho with Yossi, there are obvious similarities although Coutinho does have more pace so he has potential to become a better player.
I think it's also interesting to compare Coutinho to his fellow Brazilians Kaka and Oscar. There are a lot of similarities there, not only in their game but also in their family backgrounds: relatively wealthy, religious, hard-working and humble. Coutinho is by far the shortest of this trio and I think a bit more suited to playing on the wing than the other 2.
So far I really liked his energy and application. At this stage he might not have enough stamina to do it for full 90 minutes, but for 60 or 70 minutes he was allotted in his first 2 starts, he ran his socks off. You can see he is really trying to impress the manager and the fans and show that he can handle the speed of the English game and the defensive responsibilities of European football.
His vision, intelligence, and technique are plain to see. I think the biggest potential hurdle he will need to overcome is the stamina issue - can he maintain the kind of energy level required in the Premiership over the course of the season, while remaining mentally sharp and avoiding injuries? If he can, I'm sure he is going to be up there with Mata, Silva and Cazorla as one of the PL's best playmakers, because he clearly has all the other attributes already at a comparable level. EDIT: Maybe only his shooting is still a level or two behind someone like Mata - but even if it is (and we need to see more of Coutinho to really judge that), he can still improve our attack by a lot.
A lot here depends on the club; they need to design a carefully thought-out training and nutrition program for Coutinho, maybe not quite up to the level of sophistication that Barca did for Messi, but with a similar idea. There is a huge difference in value between even an incredibly gifted playmaker who has to be subbed off after 70 minutes in every game and gets injured a lot (the Pablo Aimar type), and someone like Messi or Suarez who pretty much never needs rest. Coutinho is a good kid, I am sure he will give the maximum effort to any training program designed for him, it just has be the right one.