I can't believe that there was a time like that even for him.
It's unbelievable to think that even a very good player like him had trouble at first.
I can't believe( 信じられない) that there was a time like that( そういう時期があった) even for him. ( 彼でも)
It's unbelievable to think( 考えて信じられないが) that even a very good player like him ( あんなに上手な 彼でも)had trouble at first. ( 最初はダメだった)