"Tats" is actually "Tatsu", as in Japanese every letter is made from one to three English letters creating a sound. I actually explained all this when he signed, but "Ta-tsu" is how it is spelt-Ta or た is one letter, tsu or つ is another letter. たつ. The alphabet is on here: 
Hiragana - The Japanese Alphabet (Hiragana Chart)
As you can see from that, there is literally 
no letter in Japanese that makes the flat "ts" sound, there is always a u there, even though it is often quiet-especially to Western ears. I heard his interview and it was quiet but there. Anyone who knows some Japanese will have heard it. Why the fuck would he have a nickname that he can't spell or even say in his own language?!
So our players saying "Tats" is actually a new nickname created through mispronunciation and ignorance that he will take as just being a very very quiet "Tatsu".