First of all, we should define what a number one offensive option should do. Most first options in an offense are players who can score with a high probability. This means, using terminology of JoBS, that they have a high floor percentage, good enough to make an entire offense good -- perhaps better than average. This consists of two questions then. First, is Ewing's individual offense good? Second, is the Knicks' overall offense good because his offense is good?
The answer to both these questions is no. No, Ewing's offense is not "good" right now. His floor percentage is just 0.502 this year and his offensive rating is just 98.5 (within the context of an average offense). No, the Knicks' team offense is not good either, with an offensive rating of 104.3, as of 3/10, with the league average at about 105.9. A quick and dirty answer like this shouldn't quite satisfy you. It doesn't satisfy me.
There is more to this question than determining whether Ewing and the Knicks are better than average offensively. The Knicks have made their reputation the past few years on their brutishness, err, their defense. Ewing has been a big part of that. Centers generally are at the heart of a good defense and Ewing is a good defender, though he doesn't get as much credit as Dikembe Mutombo, who probably gets too much. With the Knicks good defense, Ewing does not have to be good compared to the league average offense. The Knicks' opponents have to score against the Knicks' defense, so Ewing's offense should be evaluated within the context of the Knick defense.
The Knick defense now gives up an average of about 102.1 points per 100 possessions, several points better than the league average, but, importantly, not good enough to make Ewing's 98.5 offensive rating look good. So, once again, Nelson's words appear to have some hint of truth, to be conservative.
This is the present Ewing, one that has been influenced by Don Nelson. Perhaps Ewing's numbers are bad only because he has been playing under Nelson and they will improve under a new coach.
This is true to a point. Last year under Pat Riley, Ewing's offensive numbers were a 0.527 floor percentage and a 103.6 rating. These were better than they are now and they were better than the team's defensive rating of 102.3. So Ewing has slipped this year, but, out of deference to him, we will blame that on Nelson. However, Ewing's offensive numbers were still below the league average last year and below the numbers he posted when he was the number one option on the Knicks' team that went to the NBA Finals. Let's look at the 1994 Ewing.
In 1994, Ewing scored on 53.6% of his possessions, with a rating of 106.2. Although these numbers do not look substantially better than they were in '95, they actually produced a lot more wins. This is because the league was scoring less efficiently in '94 than they were in '95 (the three point line was still far away in '94) and, more importantly, the Knicks had one of the best defenses in history, allowing teams to score only 96.5 points per 100 possessions. In fact, some said that the Knicks were the reason the league moved the three point line in. On that '94 team, Ewing was a dominant center because his offense was relatively good compared to what the Knick opponents had to score against. Compared absolutely to the other centers in the league, like Shaq, Hakeem, or even Rik (doesn't have quite the same ring to it as those other guys, does it?), Ewing's offense was relatively poor and he shouldn't have been the number one option. But on a team with a defense that dominant, he was the Knick with the highest floor percentage and one of the highest offensive ratings and it was enough to compete for a championship, a championship they really should have, could have, would have won.
But that was two years ago. Now New York's defense is six points worse. Perhaps Nelson is to blame (heck, the Knicks blamed him for everything, why can't we?). But last year under Riley, the defense had just about the same numbers. Within the context of the Knicks' current defense, no, Ewing cannot be considered as a first option for a championship team.
Nelson's question, however, was about Ewing's offense, not his entire game. Ewing's offense, by my numbers, has never been his biggest asset to the Knicks. Through the years, his floor percentage and offensive ratings have fluctuated about the league average mark. During the Riley years, they were mostly above the league mark. This year, they are mostly below it. These are indicators that Ewing is not the offensive impact player he has been touted to be, but not that he hasn't been an overall impact player. His defense has helped make him a star (playing in NY hasn't hurt either) and helped to make the Knicks the great defensive team they have been. His offense, while never great, has been good enough to make the Knicks consistent contenders for the past seven or eight years, though.
Finally, I should mention that my offensive numbers do not account for one characteristic of basketball that may help Ewing. It is something I plan to account for soon, but it will take some measurements. This important characteristic of basketball is that the performances of the players on the same team are highly correlated. If Ewing is having a good day, the defense is going to sag off the other weapons and focus on him more. Hence, the players left open more should also do better. I have enormous evidence that this occurs, besides just this logical explanation. Unfortunately, I have never measured it. My suspicion, actually, is that the players who drive the offense are less correlated with the rest of their team than the supporting players are with their teammates. This measurement would be a good indicator of whether Ewing has driven his team and, if so, it would indicate that he is still a good first option -- since the Knicks have won and, despite the Nelson firing, are still winning.