Santana was the only possible choice. Stll, it's a sad commentary on the way pitching is handled these days that NOT ONE PITCHER in baseball reached 20 wins or went over 240 innings (Arroyo reached 240 and a fraction).
While I was thinking about Santana's no-decisions I figured what the hey, let's look at the National League's Cy contenders among the starters. Cy winner Brandon Webb could well have ended 2006 as a 20-game winner himself: he had eight no-decisions, pitched well enough to win in four of them, and he also pitched five complete games (including back-to-back shutouts; he had three shutouts on the year), which probably had a big factor in his winning the award as well, and he pitched eight full innings in six other games.
Chris Carpenter also had nine no-decisions and pitched well enough to win five of them. (He probably blew his shot at the Cy Young with five out of eleven starts down the stretch in which he surrendered four or more earned runs, including his final two regular-season starts.) It might have left him a 19- or a 20-game winner otherwise. Carpenter also had four complete games (three of them shutouts) and pitched eight complete innings in six other games.
Roy Oswalt had ten no-decisions in 2006; he pitched well enough to win only four of them. He also had one complete game during the season (a loss, to the Chicago Cubs) and he pitched eight complete innings in seven games, during two of which he lasted as late as a third of an inning longer before the Astros went to the pen.
I personally think the five-man rotation has done at least as much if not a little bit more to cut back on 20-win seasons as the six- or seven-inning start and the pitch count has done, but that's just me. And while you could probably isolate a small number of reasons for the five-man's advent, I'm convinced it traces to Sandy Koufax---I'm convinced Koufax's retirement after the 1966 season, at the absolute top of his game (he'd gone 27-9 with a 1.72 ERA and 312 strikeouts) and at age thirty but with an elbow condition for which there was no surgical corrective at the time, scared the hell out of enough baseball people that, little by little, piece by piece, they began to wonder just how many more arms were liable to get blown out under the kind of work load Koufax had taken on in his top-of-the-line seasons.
(To be borne in mind as well: Koufax had a somewhat extensive history of pitching on even shorter rest, including a 6-2 lifetime record when pitching on two days' rest---in fact, in back-to-back seasons, 1965 and 1966, he'd pitched and won the pennant clinching game on two days' rest, including the season-ender in 1966, not to mention throwing complete game shutouts with two days' rest between them in the 1965 World Series.)
I could be missing other factors as well, but I'm convinced that the graduation away from the four-man rotation began to plant its seed after Sandy Koufax retired.