I'm sure people will get upset about this, but I think how Southgate used him yesterday and how Jose used him against Everton are appropriate. He's not a dynamic playmaker, but what he does provide is a constant outlet and option when in possession. He's always moving and making himself available which, when you're struggling to get hold of a game, has value.
It's not always about getting in a superior attacking threat, but that at times bringing in a player that will simply allow you to hold onto the ball adds a lacking dimension. Mata is better at this than him, but Lingard is quicker, more energetic and better at pressing and tracking back as well, so late in games I can see why Mourinho views him as a viable option.