The 24-year-old signed from Dutch side AZ Alkmaar for around £6 million in the summer.
While he has played in 10 of 11 Premier League games this term, the Saints are at the bottom of the table.
Martin stated, per the Daily Echo: "I think he's been outstanding. He has adapted to a different culture and different environment.
"He did not have his family here for the first couple of months as well and the way that he just dealt with that was impressive.
"They have come over now so Yuki is smiling - he smiles a lot anyway. I think he's been fantastic.
"The only thing that sort of disrupted his season a little bit was he took a knock against Leicester just after the international break.
"With the amount of traveling he does (to play for Japan), he went away and didn't play, which makes it really difficult to come back with freshness and energy.
"I think he's been amazing, he's going to be a brilliant player for us. He's ready for the Premier League and he's done great for us.
"He's been so unlucky not to start but he came on and made a big impact (against Everton), so now he needs to continue doing that."