Loku is undoubtedly a decent cricketer on the Sri Lankan domestic circuit, but I'm afraid on this showing he's nothing special. He's one of those new breed of 'spinners' you get from T20, they roll the ball out and spear it in, are afraid to give it any work or flight.
He may have got two wickets against Pakistan today, but one was a long-hop pulled to short mid-wicket and another was a straight ball which Umar Akmal played all around. He's played a couple of Tests and 21 ODIs before, but the fact that he only has five wickets from his four Tests sums him up: no penetration. He finished with 2-30 off his four overs, respectable figures in T20 normally, but given that few players managed to get in on the pitch, he surely would be disappointed, especially given the three consecutive boundaries he gave away from his last over.
The main problems with Loku are that he doesn't 'explode' through the crease and just rolls the ball out of his hand, meaning that despite bowling at a pace quicker than most spinners, he's pretty gentle. Shane Warne's leg-break used to fizz out of his hand, Loku's ambles apologetically.