The England defender is expected to be out for around eight to 10 weeks but Martinez revealed he's already on the right track.
Martinez said: "John had his surgery on Monday and it was very successful.
"Now we are starting to look at counting the days until he will be fully fit again.
"John has responded really well to the last two or three days of work in terms of healing the ligament. He has made a good start to his recovery."
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