"That same cheerful, outgoing demeanor, however, has masked an internal struggle in the past, one that he confronted in 2016 and told the world about this summer. Mangum has been living with mild depression and anxiety, and reached a point last year where his parents convinced him to seek help. He learned the tools to cope with the illness through counseling and began taking antidepressants."

I have never taken antidepressants, but from talking to people who have, it sounds like they take off your mental edge and make you less intense, less caring, less motivated, basically more 'meh'.

This seems to me like the best explanation for Tanner's regression. Off the field, antidepressants would probably dull any sense of urgency or responsibility when it comes to studying and mastering the offense. On the field, they would probably slow down the brain's ability to read defenses, quickly check options, and make instantaneous decisions.