The rule is based on the initial phoneme or sound, not the letter itself. Just as @EthnicFury says, in this case it would depend on whether you would read it as running back or R-B. You're right about the vowel/consonant thing, but it's not the letter that matters, but the sound.Nope, you’re wrong. it’s whether the following word starts with a vowel or not. If the following word starts with a vowel, it would be “an apple“, or “an igloo“, or “an expert” , or “an ox“. If following word doesn’t start with a vowel it would be “a running back”, ”a beast”, ”a truck”.Henry is just part human, he’s a cyborg.The problem is, when an RB gets "old", he gets old fast. Yeah, Henry might still have some left in the tank. Maybe enough for a top 10 finish in 2023. Or he could start to hit the cliff halfway through next season. But if you're in keeper/dynasty... you have to be looking at moving him for your next 3/4 year stud while he can still get you that kind of value.
Also, it’s “a”, not “an” RB If next word starts with a vowel it would be “an”.
If we’re gonna be pedantic It’s actually more based on the sound than the letter, so it depends whether you’re writing to be pronounced “are-bee” (which would get an ‘an’) or “running back” (which would get an ‘a’)