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Vincent Kompany has revealed "a number of small and local injuries" are hampering his comeback.
The Manchester City captain came on as a second-half substitute in the 4-1 win over Sunderland following an eight-game absence, but lasted just nine minutes before a recurrence of a calf problem forced him off.
Kompany admitted his fitness problems were a source of frustration, but vowed to return stronger than ever.
"Today was a bad day for me," the 29-year-old Belgian wrote on his Facebook page.
"My body feels invincible - the best it's ever been - but i'm challenged by a number of small and local injuries. It leaves me more frustrated than with any of the 'big' injuries I've had but it's a challenge that I will overcome.
"There are worst things in life of course and I never forget that. So I'm definitely not asking anyone to feel sorry for me and I hope you wouldn't anyway. This is a way to communicate my situation to those who maybe deserve answers.
"I've made many sacrifices to get where I am today and football is everything to me. This is what I fought for so hard. I want to be the best defender in the world and I want Manchester City FC and Belgium to become the best club and country in world football. I won't give up that dream. Injuries have and still are preventing me to play my role the way i'd want to but it's not stopping me to progress. I will get on with it and when I do come back.. I will be stronger again.
"My mindset stays unchanged, I will fight to get what I deserve, my best years in football."