This watch is as good as, if not better than, the watch I lost .

这是把省略了成分的条件状语插入到主句中间了。等于：

If this watch is not better than the watch I lost, it is as good as the one I lost. 

这种结构其实还是比较常见的。这样安排句子，主句和从句共用the watch I lost.句子更加简练，避免了无谓的重复。

翻译：这块表即使不比我丢的那块更好，也至少是一样好的。