Manchester United defender Phil Jones has revealed the extent of the problem which saw him miss out on the start of the new campaign after he suffered from thrombosis during pre-season.
The England international was not able to take part in any contact training throughout the first few weeks of the current campaign because of fears that a blood clot in his calf would spread to other regions of the body.
After making his return from the bench against Ipswich Town in the League Cup last week and featuring for 17 minutes as a substitute in the 3-0 win over Sunderland on Saturday, Jones revealed his relief at finally getting back to playing football.
"The blood clot was something that was completely out of my control," he told reporters. "It wasn't a tackle or something stupid, it was just one of those things. I came off the pre-season tour of America and felt a tightness in my calves and had a scan on it.
"I was still fit and still mobile about the house and that was the frustrating thing, still walking around and trying to train but I couldn't join in with the lads, I couldn't get contact on it, that could disperse it and then you are in trouble.
"So I had to train alone on doctor's orders. I saw the specialist and his advice was not to train with the others, so when things like that happen you can't really ignore it. Now I am fit and fine. Touch wood I can stay fit and make my way back into the side."
Jones has played 130 games for United since signing from Blackburn Rovers in 2011.
