The chain of custody associated with urinalysis, especially for the NFL, is bullet-proof. Not only is the sample going to be authentic, but the samples are chemically tested for reactivity and are put through a gas chromatograph. The technician performing the analysis does so under both audio and video surveillance, and all positives are re-run multiple times for confirmation.
His body contained the substance, and unless he was held down and forcibly injected by some mafia operatives, or was the victim of food-tampering of some kind, he knowingly ingested the illegal substance.
Just own it, Welker, and work on your future.