I want to elaborate more on Mo’s point here. Specifically, I want to highlight what Griezmann can offer Barcelona, as I’ve seen many say he does not fit.
Firstly I want to talk about his defensive contributions. He was usually the man furtherest forward at the start of his Atleti tenure, but gradually he has been given a bigger responsibility defensively.
vimeo.com/219098622
vimeo.com/219098622
Griezmann has fresher legs than Suarez, at this point is better at pressing opposition defenders, & also tracks back should the opposition break Barça initial high-press. Messi would thus be the only inactive player defensively, & the furtherest man forward when defending.
Secondly, Griezmann is a better at progressing the ball than Suarez, especially in counter-attacking situations. When Barça win the ball back deep in their own half, Griezmann is capable of bringing the ball forward and finding runners, or combining with Messi.
Griezmann would offer a lot of versatility in counter-attacking situations as he is capable of making runs in behind as the true #9, play out wide to stretch the opposition, or drop deeper to initiate the counter himself. vimeo.com/219098360 
In an organized attack, Griezman can operate as a shadow striker, inside forward, or a traditional number “9”. As this video shows, Griezmann is adept at all 3 roles. This versatility will add a lot to Barcelona’s attack.
vimeo.com/219098423
vimeo.com/219098423
With Messi playing centrally & Griezmann pushing the defense back, Barça can create numerical superiority centrally, giving Messi the space to create, dribble & shoot around the edge of the box. If a central defender steps out, a diagonal run by Griezmann will punish him.
Furthermore, should a wide defender be dragged into the middle of the pitch to help compensate Barça’s superiority in the middle, Dembele or Alba will be free on the wing, giving Barça a very balanced attacked.
Griezmann could also drop deeper and drag a defender with him, giving Messi space to make a small run into the box. With the midfield we will have next year, as well as Griezmann’s creativity, Messi should be able to get more chances in, and around, the box than he does now.
Finally Messi and/or Griezmann are capable of drifting wide to overload either flank to help break down the opposition through the sides, & the other occupying a striker position inside the box. Both are also extremely capable of switching play to wide player on the other flank.
More than anything Griezmann will add versatility to Barça’s attack. Perhaps his best position is similar to Messi’s, but both Messi and Griezmann’s true value comes in their versatility and intelligence. This is where Griezmann will help.
Having a fluid attacking system such as this will keep opposition defenders guessing, as both Messi and Griezmann will be able to play different roles throughout the season, and even within the same match. Griezmann will also compensate for a static Messi defensively.
In order to truly get the best out of this duo, a manager that adheres to the principles of positional play is needed. Setien/Tuchel/Ten Hag will be able to help instruct both players, on where to position themselves depending on their teammates & the opponent.
Even without such a manager I believe Griezmann will add a lot. But in order to make the most of our versatile front-line, we need a fluid system where out forwards can interchange positions and roles. Appointing Setien/Tuchel/Ten Hag is our best bet to create such a system.
Allow me to end this thread by saying #ValverdeOut
