None of us are in a good position to evaluate the merits of his denial. I accept at face value that banned substances were detected in sufficient quantities as to fail the test.
The problem is, it's my understanding that part of the agreement (between the league and players) is that they are prohibited from disclosing exactly what substance was found. Hence, it leaves us guessing at the possible sources of inadvertent ingestion. Medical test results are scrupulously guarded, protected by law under HIPAA, so it will take a very intrepid reporter indeed to ferret out the triggering substance. Without knowing what it was, we speculate so wildly that there's really no benefit to doing so. (It was his eye drops!!) etc.
I'm guessing that since he was unable to identify what it was he ingested, he doesn't have a ready alibi. I would feel a lot better if he showed us his bottle of GNC nutritional supplements and a receipt from 3 months ago, and could demonstrate a link to the identified substance.
If I had to guess, I'd say his lack of alibi tells me he's probably guilty, and just wants to get on with things. But that's just an uneducated guess.
As an interesting aside, I attended my first Orioles game of the season Friday night, guest of a business associate in a very nice suite. I made a comment during the game that Raphy looked smaller to me than I remembered him. Guess not small enough.