@Tetsujin reported last year that this was expected. He's making headlines for signing a MLB contract directly out of High School -- a move that Shohei Ohtani also considered (the Dodgers were close to getting him at the time) but ultimately chose not to make. Ohtani made his MLB debut at age 24 after spending 5 years in NPB. Morii is an exciting prospect to follow for the next few years but he is just 18 years old. Based on a typical minor league development timeline, it's unlikely that he makes it to the big leagues while the A's are playing in Sacramento. You're more likely to be able to watch him in Stockton in a couple years in High-A ball.
I'm always excited whenever the A's sign a player from Japan but he's going to have to overcome a lot of bad history to succeed with this organization:
None of the high profile international teenagers that the A's have signed to big money deals in the Moneyball era have panned out so far, most notably P Michael Ynoa, SS Robert Puason, and OF Pedro Pineda who all brought top of their class pedigrees and earned huge multi-million dollar signing bonuses but ultimately went nowhere. To find the last big success story for the A's in this area you have to go all the way back to Miguel Tejada in 1993.
The biggest Japanese free agent signed by the A's was SS Hiroyuki Nakajima (an 8 x All Star with the Seibu Lions of NPB at the time) who signed a 2 year $6.5 million contract in 2012. In his press conference he said, somewhat tongue in cheek, that he chose to play for the A's because "Billy Beane is sexy" -- I still remember this because I watched that press conference. The Moneyball movie starting Brad Pitt as Billy Beane had just come out the year before. Hiroyuki struggled in Spring Training though and never ended up playing a single game in Oakland before returning to Japan.
