Depends what you mean by "greater." By just about any definition, I'd say no - or at least it would require more than just that. First of all, people focus too much on Slams. Yes, they're the big prize in tennis and most regard Slam title count and career rankings as being synonymous. But I don't think that's the case - or at least is a huge over-simplification.
There is no clear way to weigh this, but what if we viewed Slam title count as 50% of greatness, and everything else as the other 50%? That's overall titles, rankings - weeks at #1 and year-end #1, as well as top 5, 10, 20 finishes - big titles, tour finals, win %, best years, head to heads, Slam results other than wins, etc. It is very unlikely that Rafa's "other 50%" will ever match Roger's. In particular, I would say the second most important factor in greatness after Slam titles is rankings, in particular weeks at #1. Consider what a week at #1 means: that a player had the best previous year on tour. Roger has 310 such weeks in which he was the best player for the previous year; Rafa has 196 - that's quite a gap, over two years. If Rafa doesn't win the AO, he is probably unlikely to ever reach #1 again (Novak has few points to defend over the next few months, while Rafa a ton). Rafa will finish with more big titles and might even narrow the title gap to very little, but may never win a tour final.
Now if Rafa equals Roger's Slam count and returns to #1 for at least a year (to finish with 250+ weeks) and throws in a few other perks (e.g. more big titles, an ATP final or two, etc), then I think he'll surpass Roger's resume. If he wins 22, 23 Slams to Roger's 20, it would be hard not to consider hiim greater.
But all of that is about career accomplishments. Another way to look at "greatness" is far less tangible or quantifable. It is more the quality of the player and to what degree they embody "greatness." In that sense, Rafa may already be Roger's equal. Some might disagree, but when I think "greatness" I think Roger, Rafa, and Novak. In my mind--and in this way of looking at it--they are all GOATs. It doesn't even matter what their final career resumes look like - they're already the best of the best. They all embody greatness, or different versions of it.
Consider the question: Who is the greatest rock band of all time? We can look at the equivalent of "career accomplishements:" albums sold, number one songs, etc etc. But in the end, whether we're talking the Beatles, Rolling Stones, Pink Floyd, Led Zeppelin...there is a subjective factor of taste and preference and style that comes into play.
Now sports isn't quite as subjective, but I'm merely pointing out that we really can look at it in at least two ways: quantitative (as represented by career accomplishments) and qualitiative ("greatness" as a more intangible, subjective sense) - and both involve subjectivity (how you weigh records and stats on one side, and what qualities of greatness you emphasize).
All that said, there is still the possibility that Rafa or Novak separate themselves from the pack in terms of overall career resume, and make us forced to accept one or the other as the singular GOAT. Right now Roger has the edge, but the gap is narrowing. In a way we're back to 2014 or so. For a bit there, in 2017, it seemed like Roger had pulled away. Now it seems less clear. But for Rafa to clearly surpass Roger, he needs to at least win 20, and he needs to pad just about everything else. Another 50+ weeks at #1 and a WTF would do a lot of good to his resume. For Novak, just more of everything (Novak might have a better chance because not only is he a year younger than Rafa, but he doesn't have the big gaps that Rafa has).