Only a few hours earlier Michael Jackson had been rehearsing at a local sports stadium，but concerns about his fitness for the concert tour had already begun to circulate，amid reports that he was looking frail.
这是一个并列复合句，即“第一分句+ but + 第二分句（之中含有同位语从句）”，具体分析如下：
第一分句：Only a few hours earlier Michael Jackson had been rehearsing at a local sports stadium，
时间状语：Only a few hours earlier 只是在几小时之前
主语：Michael Jackson 迈克尔·杰克逊
谓语动词：had been rehearsing 已经在排练着
地点状语：at a local sports stadium 在本市体育场
第二分句：but concerns about his fitness for the concert tour had already begun to circulate，amid reports that he was looking frail.
主语：concerns about his fitness for the concert tour 对他旅行演出健康状况的关注
谓语动词：had already begun 已经开始（already 是插入性状语）
宾语：to circulate传播
状语：amid reports that he was looking frail. 在他看上去十分虚弱这种传言中（修饰circulate，其中that he was looking frail 是对reports 的进行具体说明的同位语从句）
翻译：迈克尔·杰克逊仅仅在几小时之前已经在本市体育场排练着，但是对他旅行演出健康状况的担忧已经开始在他身体显得虚弱这种传闻中蔓延开来。
追问我感觉这样翻该要好些吧：仅仅在几小时之前迈克尔·杰克逊还在当地的体育馆彩排，但在他身体日渐虚弱的传闻中，对他巡回演出期间健康状况的担忧早已经流传开来了。
追答可以。