The Scotland midfielder was out for two months towards the end of last season with what was described as a stomach virus, then failed to begin pre-season training on schedule after appearing to suffer a relapse.
"We never got to the bottom of what the virus was but it knocked me back," he told the Daily Mail. "I'm hardly a person who can afford to lose weight but I did.
"Now though I'm fully on the mend. I missed the run-in to the championship and a place in the starting line-up for the Champions League final.
"It was scary, but actually with the diet I'm on now I'm actually heavier than I was. I feel fighting fit."