Grand Slam legend McEnroe is worried by the nature of Murray's injury. He told The Independent: "I don't know exactly what's going on with his back, but obviously one second it seemed like he was holding it and the next second he's sprinting.
"If anything, people are saying the gap is getting bigger, not smaller, so time is becoming of the essence. It almost seems like he's just got to get that mental part of it where he's got to forget about it.
"He's got to say I don't care if my back is hurt, I'm not going to give it off. That's what he's got to think about if he's going to win Slams. These guys are tough to beat, really tough."
Murray will be hoping to chase down that elusive major championship at Wimbledon, which begins in just 17 days' time.