I find this talk quite interesting so will add my two pence!
Given Dembele's, Lamela's, Dier's, Walker's and Rose's improvement this season, not to mention the whole team ethic that has occurred, I just don't get how anyone can say with any true level of confidence that Mason, Carrol, N'Jie and any other player should or should not be sold/traded/upgraded. People were saying that we should sell Dembele, because although he had potential, he would never fulfil it. I am a Mason fan, and no doubt he has a job on his hands to win back his place but I'm not convinced that this is beyond him.
I also don't subscribe to the theory that getting in 5 players will be too much and will mess up the harmony in the squad. What this season has shown is that we have avery strong first XI but we don't have as strong a squad to compete fully in the cup competitions and Europe, meaning that Poch did have to prioritise some competitions over others. With increased money from TV and hopefully CL, we should be able to do that and 5 extra squad players could be just what we need. However, Poch knows best, and I am sure that we will back him in the market and get the right players. To have someone like N'Jie could be like having a new player given the limited time that he has played.
I think we would benefit from a striker, some competition for Dier, a backup goalkeeper (as I agree with Dubai that from a financial perspective, now may be the right time to cash in) and another midfielder. Some of those positions can be filled by the academy, but I do think that we could benefit from a real leader. Someone that our youngsters will be in awe of and will learn from.
The most important point though, is that I think every single player in this team has the capacity to play better. Learning how to grind out a 1-0, having players that make a difference when coming off the bench and also having the ability to play with two strikers will mix things up and mean that we can get the extra points that will mean a league title. Ultimately Poch will decide, but in my mind the most important signing that we can make this summer is Poch himself. He is instrumental for our team and it is obvious that a lot of these players have a great loyalty to the man.