I don't think they are looking at Tazawa as a starter, even though I see the appeal. He does have a range of pitches and he controls the game well. He has a lot of poise. Let's face it though, great Japanese starters have been rare in mlb. They are generally not a large race of people. They do not usually have that 96 mph fastball and up and their training usually involves throwing a ridiculous amount of innings before they even reach the majors making them a greater injury risk. History has not been kind to Japanese starters in mlb.
I understand that Darvish appears to be doing just fine but he is also an extremely large Japanese pitcher with exceptional stuff. He should be a solid starter in mlb for a long time but even his arm has a lot of wear and tear and he could implode even at some point. I wouldn't count on that any time soon though!
Tazawa had relatively little wear on his arm for a Japanese pitcher but he already has had elbow issues.
We should not fix what isn't broken. Leave him in the pen. We are going to have to develop our future starting pitcher depth with longer term options, through the draft in particular. Which we clearly are doing. We clearly have decided to focus on that approach. MLB starters are just not available at a reasonable price through free agency and via trade.