The definition is evolving these days but in the traditional sense of it, a 'Number 10' is your playmaker. The core of your attacking wheel. The main link from midfield to attack. Typically, he can play as a central midfielder, attacking midfielder, or as part of the striking cast. He creates chances, he finds spaces, he finds the main strikers. He is a good dribbler - a baller, a set piece specialist; his football IQ is high and he scores pretty well. He is your Pele, Maradona, Zico, Messi, Deco, Sneijder or Bergkamp.
Jose has used Mikhi, Mata, Rooney and Lingard as number 10s this season. One problem is that we have not had one of them own the shirt neither has any of them been good enough on a consistent basis ('consistent' being the key word).
Jose does like traditional number 10s in his squads. He has said it a few times and his comments about Mata and Oscar at Chelsea are well documented. He also used Fabregas as a 10 a number of times. At Inter, buying Wesley Sneijder won him the treble. At Porto he had Deco. At Madrid, he did everything to buy Ozil.
One suspects that with what Jose has seen this season, he'll be convinced he needs to buy a number 10.