Souleymane Coulibaly comes in higher than Ezekiel Fryers on our prospect rankings simply because he's quite a bit younger. At just 18 years old (19 in December), Coulibaly still has a lot of room left to improve and some time to turn his career around before Spurs let him loose. However, he's very comparable to the former Manchester United left back in that he was very highly touted, cost a significant transfer fee, then failed to live up to the hype early in his time with the club.
As you may remember, Coulibaly was the star of the 2011 Under-17 World Cup. Past Golden Ball winners in that tournament include, but are not limited to: Lionel Messi, Sergio Aguero, Caio Ribeiro and Emilio Peixe. Obviously, there's a bit of variation there because it's impossible to predict the career path of a 16-year-old. Some players who flop at that tournament become stars (Neymar). Some guys who dominate at that tournament never become more than average pros.
Sadly, Coulibaly looks to be on the latter path at the moment. He scored 10 goals in seven matches for the Ivory Coast Under-17 team, but he's yet to show that same promise at youth club level or in Serie B, where he played for half a season on loan to Grosseto in 2013. He's obviously a very good finisher and has some solid pace, but it's not obvious that he has or ever will have more than that.
It's also patently unfair to heap criticism on a player who was labeled 'the next Drogba' even though he's 5'7" and had only played in one real tournament when he picked up that nickname. It's not like he was banging in goals for fun for Siena's Primavera team before the Under-17 World Cup. He was just a guy. Spurs took a calculated risk on him that could have resulted in them signing a world class player for a high six-figure fee, and it looks like they're unlikely to do better than break even on his purchase.
Hopefully everything I've just written turns out to be wrong and Coulibaly becomes a star, but at the moment, there are at least 12 players in the Tottenham youth system that appear to have more promise than he does.