Here's the thing-- there are significant differences between Japanese baseball and American baseball: in MLB, you use a 5-man rotation rather than a 6-man rotation in NPB and the MLB season is 162 games over six months while the NPB season is 144 games over eight months. These seemingly minor differences have enormous implications on a pitcher's routine throughout the season. Japanese starters are usually never on pitch-counts and engage in much more rigorous off-day workouts, which they are able to handle physiologically because they will usually have six days off between starts, rather than the four days American pitchers have. Daisuke reportedly never really acclimated that well to the MLB starting pitcher routine, but this team isn't going to go to a six man rotation just so he feels more comfortable.

More relevantly, there's also the way that, in Japanese baseball, many pitchers work off their breaking pitches rather than off their fastball, as in MLB. Here's an excerpt from Michael Street at BP:

NPB pitchers also rely more on the breaking ball, either because they lack power arms or because the form-obsessed Japanese appreciate the pinpoint precision of a perfectly placed breaking ball over the sheer force of a blazing fastball. Their pitchers often master five-to-six pitches, like Hideaki Wakui, whom Patrick has charted with seven different offerings.

After filling the count, NPB pitchers will often use one of these breaking pitches on a 3-2 count, a stratagem that’s helped by more subjective umpiring. NPB umps are given a few weeks’ training and don’t have a system like QuesTec or PITCHf/x to rate them or a strong player union to demand excellence. Instead, angry NPB players often berate them, even making physical contact, leading umps to favor big-name pitchers or top teams to a much greater degree than in MLB.

The combination of these factors creates several adjustment problems for NPB pitchers making the switch to the majors. Those who succeed as starters typically pitch aggressively, have good fastballs, and adjust to the conditioning and workout schedule without incurring injury—the same tools any MLB pitcher would need. The disproportionate number of high-profile flameouts among NPB starters (or pitching prospects anywhere) shows how hard this is, although there have been some notable successes; relievers have been more successful, a subject I’ll cover in my next column.

I think this is more of what Valentine mentions specifically in that article. Certainly, there are junkball-type pitchers who work mainly with their offspeed pitches, but those pitchers are the exception and not the rule and require pinpoint command. Daisuke has never shown the command (of either his fastball or his off-speed stuff) to be able to work effectively like this, in my opinion. Daisuke's superb NPB command has never carried over to MLB-- whether due to a different, more slippery American baseball or more disciplined American hitters (Japanese hitters generally prioritize putting the ball in play over taking a walk).