"It is with regret that on doctors advice I have to withdraw from the Mutua Madrid Open with injury,” Murray said in a statement on the Madrid website. “I always love coming to Madrid so it is a big disappointment. I look forward to returning next year and wish everyone a great week of tennis.”
Disappointed but not surprised. I did wonder what was going on when even am.com were silent as to what Andy was doing and hadn't changed the name of his next tournament to Madrid.
However, according to the above report it's stated: "Britain’s Andy Murray, the world number four, will miss the Mutua Madrid Open after suffering an unfortunate back injury
". That doesn't sound to me like it's an old or ongoing problem but something more recent. Did he pick it up in Barcelona, training in the gym, practising on the (slippery?) blue clay, or something unrelated to tennis? As usual lack of more explicit information means his fans will be left to speculate.
I could be proved wrong, but I feel, and hope, that this is a precautionary measure, given that Andy wasn't happy about playing on the blue clay anyway, so why aggravate a problem before Rome which could affect his appearance at RG?