Let's see, Del Potro had wrist surgery and might return by the end of the year.
Nole had wrist strain in Monte Carlo and is on full rest orders for at least 10 days.
Wawrinka just won the Monte Carlo Masters. He is entered to defend his title next week in Portugal, but it is not certain he will play there, or wait until Madrid to play next.
Berdych is healing a strain, I believe, or is just resting, and is scheduled to return in Madrid.
Gasquet hurt his back, and withdrew from Monte Carlo. I heard he might ask for a wild card in Portugal next week if his back improves.
Isner is resting his shoulder I believe, though I can't confirm that, and he is scheduled to return in Madrid.
Raonic is scheduled to play next week in Portugal.
Dimitrov is playing in Bucharest as top seed this week along with Youzhny, Monfils, Seppi, Pospisil, Simon, and others, so that splits the player field between there and Barcelona this week.
That leaves Andrew Murray, OBE. I have no knowledge of Mr. Murray's whereabouts, but I think he skipped Monte Carlo to heal a few niggles, and get in more practice, and continue his search for a coach, and is not scheduled to return to the tour until Madrid, though I also heard a rumor that he might take a wild card in Portugal.
Respectfully,
masterclass