How so? Pogba isn’t played at the tip of the diamond so I don’t see why he would need to be sold to accommodate Coutinho in that formation. Ideally it would be one or the other (diamond midfield or no Pogba). There is a position in midfield at Barcelona that he is suited to but they play him out of position instead.
I think there is an argument for having both in the same midfield three, though. Coutinho is perfectly suited for the left-sided midfield position, and most advanced midfield role, in a three. He is very much a grafter, too. Pogba is much more comfortable operating from the right than the vast majority of right-footed, predominantly left-sided players, and as an attacking player in midfield he looks to deliberately play to a side. I’m not really aware of anyone who we could bring in for the right-sided midfield position who would be comfortable and effective operating from that side of the pitch in an attacking sense to balance the team. So although Pogba does seem to favour the left, playing him from the right could benefit us overall.