I mean yeah, once you've read Slam Dunk almost all manga seems inferior, sports manga included, since it's one of the greatest manga of all time.

But that doesn't mean that there's no value in other manga. Like, I believe the Beatles are the greatest band of all time, and I still enjoy listening to other bands. And I believe Hendrix is the best guitarist, yet I enjoy other people's guitar work, too. Godfather I&II are the greatest mob movies of all time, but there are still a bunch of crime movie masterpieces out there, all worth watching.

And so I believe Slam Dunk is the greatest sports manga of all time (by far) and one of the greatest manga of all time, but I can enjoy other manga just as much as I did before reading Slam Dunk.

Your problem is not actually a problem. Eyeshield 21 is a great series. It's also nothing like Slam Dunk, Slam Dunk is mostly a high school delinquent manga where the characters play basketball, and Eyeshield 21 is a comedy/exaggerated-reality manga where the characters play football. I don't think that the fact both manga center on a sport makes them at all similar to one another.