According to my experience training depends on the age not stars ...
You can power train him ( use personal training to boost him 4 % for 64 tokens * 4 times to reach 100% or spend so many rests ...)
So look at his age if he is very young ( below 22) and talented ( positions - skills ) do this investment if you are in dire need of him ....

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If it was my decision , i won't buy him because he would cost me ( 10-20 tokens + around 50-100 rests + time ) ....