The Japan fullback signed for Saints in the summer from AZ Alkmaar.
Martin said, "I think he's been outstanding. To adapt to a different culture, different environment, to not have his family here for the first couple of months as well and the way that he just dealt with that.
"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.
"After the international break, with the amount of travelling, he does and then he went away and doesn'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 and he was so unlucky not to play at the weekend but came on a big impact and so now he needs to continue to do that."