[NOTE: I do civil and business litigation, and generally avoid criminal court like the plague. This is my initial reaction and that of some people I know who do this kind of work. There are lawyers who read this site who likely have more information on how this could play out than I have. Feel free to correct or supplement this in the comments; you won't hurt my feelings.]
He was formally charged with six counts of drug charges — unlawful possession of a prescription drug (two counts), one count of possession of controlled substance (marijuana), one count of possession of dilauded (a controlled substance), one count of possession of cocaine and one count of Diazepam. He also was charged with keeping a drug house (an aggravated misdemeanor. Johnson-Koulianos offered no comment as he left the Johnson County Courthouse.
From what I can tell, only the drug house charge is the only aggravated misdemeanor, with the other charges below that level. An aggravated misdemeanor can be punishable by up to two years in prison (that would be state prison; county jail is for one year or less) and fines up to $6250. Possession of a controlled substance is typically classified as a serious misdemeanor under Iowa law, carrying a maximum penalty of one year in county jail and up to $1875 in fines. Marijuana is its own animal, with a six month maximum jail sentence and potential fine of $1000. These are serious penalties, and these look bad on paper.
In practice, though, DJK is highly unlikely to spend any more time in jail than the time he served last night. His record is relatively clean; since arriving at Iowa, Johnson-Koulianos has the litany of traffic violations typical of any teenager but nothing significant on his record and, given his age, anything he may have had in Ohio is likely disregarded as the minor misdeeds of a juvenile.
Further telling of DJK's legal position is the drug house charge. Had he been involved in the sale or distribution of drugs to others (as his roommate is reportedly alleged to have done), Johnson-Koulianos would have received more than the aggravated misdemeanor charge. One gets charged with "keeping a drug house" when there is no evidence he or she is directly involved in the distribution process, but rather fails to do anything about the guy in his or her house who is doling out dime bags. Given what we know about the other charges (no possession with intent to distribute on any count) and the roommate's charges (which are more significant, by far), this makes sense. With six charges, none of which were able to rise to the level of a felony, and no prior record, DJK is likely to get probation and/or community service.
This fate is made even more likely if he keeps talking. The target of the police raid wasn't Johnson-Koulianos. It probably wasn't even the roommate (the $3000 cash doesn't exactly make him Tony Montana). It was probably the roommate's distributor, or that distributor's source. The roommate is small time, and DJK is even smaller than that. Some were probably correct to criticize him for talking to the police immediately after the search warrant was executed, but if the drugs were found and he was going to fail the urine test, best to keep your credibility in check for a potential plea deal. In that circumstance, honesty might well be the best policy.
He'll never play for Iowa again; given Ferentz's track record with this kind of thing, he would likely be kicked off the team next week even if he wasn't a senior preparing for his final bowl game. He's likely done irreparable damage to his potential career in professional football. He's put an incredibly apropos punctuation mark on the end of this disappointing season. But, as far as his legal problems go, DJK could certainly be doing worse.