Firstly, Poba does not and will not be playing behind the striker/CF! Pogba will be the CM part of the DM+CM in a 4-2-3-1 or the LCM when we play 4-3-3. He will be the central play maker as a CM and not an AM. That is how Mourinho has been deploying him and he has been adapting quite well. The rest of the team will be built around Pogba's ability and role.
Secondly, James is a level above Mikhi and 2 levels above Mata. He has a very high football iq and can play any of the 3 spots behind the lone striker in a 4-2-3-1 (or any of the attacking trio in a 4-3-3). Good at set pieces and a great finisher (though not as good as Griezmann but better creative wise and more rounded). A simple look at his goal and overall play today shows his quality. He also works hard for the team (and one can see traces from his stats like distance covered, tackles, ball recovery etc)
He is captain of the colombian NT. He was the highest goal scorer at the last world cup, top assist in ligue1 (in his only season at Monaco) and best midfielder in la liga in his first season at Madrid. His recent problems are more on the failings of Zidane (who plays an injury prone Bale and clueless Vasquez ahead of him, but then Zizou also leaves Morata on the bench and doesnt know to defend a draw when they are down to 10 men
). He is a proven world class level player and is only 26 (a few months younger than Griezmann).
His only problem is his injury record but his price tag would be less than Griezmann. You might not like the kid but try not to belittle or misrepresent his abilities.
PS: Sample stat comparison of James and Griezzman on squawka
http://www.squawka.com/comparison-m...es_won/successful_take_ons_%/interceptions#90