Srna has many of the qualities that Benitez clearly admires. As you may have seen in the World Cup he has a good deal of pace and is very determined. His commitment is excellent and against Japan he was substituted towards the end because he had run himself into the ground. This was after he had missed a first half penalty (don't worry, it was a good save rather than a poor penalty) but was focused enough not to let it bother him and still give everything for the team.
For his club, Srna often plays right-back but has clearly expressed a preference for playing on the right side of midfield. Various reports suggest that he would consider a move because of this and now because he has proved his talent, he surely belongs in a bigger league than the Ukrainian championship. However, his versatility would not have been lost on Benitez who admires Daniel Alves for the same reason.
Okay, so he's quick and enthusiastic you say but what else? Srna also possesses a great free kick and a good eye for goal. He has scored some spectacular free kicks for his club in the last couple of years and scored five goals out of nine World Cup qualifying appearances - an excellent record for a midfielder.
There have been a couple of reports linking him with Liverpool in the past, so hopefully Benitez is at least considering him for the club. One thing that would go in our favour is the lower transfer fee he would command than those other players previously mentioned. Shakhtar are quite a wealthy club and are not desperate for the funds but if Srna wants to leave and they were offered £5-6M they may reluctantly take what would be a good return on their outlay for him three years ago.
One thing that may keep the transfer fee at a sensible level is the fact that he has been a solid but not star player in the World Cup and the fact that Croatia are unlikely to progress to the knockout stages so his value shouldn't be inflated too much.
If the Alves deal is completed then it's unlikely we will get Srna too, although Pennant was being talked about as cover in this position and he would cost £4-£5M. For an extra £1M I know who I'd rather have in the team.