I think it is always unwise to underestimate your opponent/competition, and I think Harbaugh and staff must assume Day will be a great coach starting next year. One thing that I keep hearing, however, is that OSU has found its Lincoln Riley. I, being the nervous fan that I am, decided to take a closer look at their histories when it came to points per game at their previous stops (OC and HC only). I understand that these numbers may include some defensive and special teams touchdowns and that I don’t know rely on the fancy S&P stats, but I think they provide some data when people make this specific comparison.
Ryan Day’s resume:
OSU OC under Urban Meyer (2017 and 2018): Averaged 41 and 43 points a game respectively (Great Offense). He did give OSU a bump in scoring from the 2015 and 2016 seasons (35.7 and 39.4 respectively), when Warinner was the OC, but their offenses averaged more points when Herman was the OC in 2013 and 2014 (44.8 and 45.5 points a game respectively), despite not having the benefit of Urban's recruiting dominance. He also had Kevin Wilson as his Co-OC, which he'll likely have anyway.
BC OC under Steve Addazio (2013 and 2014): averaged about 27.7 and 26.2 points a game respectively (which is meh). Addazio has more of a defensive approach, so perhaps they were going for body blows instead of head shots. After departing, BC's offense was putrid and averaged 17.2 points per game in the year after his departure. Once Loeffler joined the staff, however, they went 20.4, 25.7, and 32.0 points per game over the next three years.
Temple OC under Steve Addazio (2012): averaged 24 points a game. Again Addazio was the coach here, so maybe going for body blows here too. In the two years preceding Day, Addazio's teams averaged 25.0 and 30.6 points per game. In the four years after Addazio and Day left Temple, they averaged 24.9, 23.1, 29.8, and and 32.4 with Matt Rhule at the helm.
Lincoln Riley's resume:
Oklahoma HC (2017 and 2018): 45.1 and 49.5 points per game respectively (and virtually no defense)
Oklahoma OC under Bob Stoops (2015 and 2016): 43.5 and 43.9 points per game respectively. Obviously, Stoops was the head coach. Prior to his arrival, OU's offenses averaged 38, 32, and 36 points per game in his previous three years.
East Carolina OC under Ruffin McNeill (2010-214): 36.8, 26.2, 31.5, and 40.2, and 38.8. Ruffin McNeill was the head coach. The year he left, they went back down to 27 points a game with McNeill still as HC for one more year. In the year before McNeill and Riley arrived, East Carolina averaged 27 points a game under Skip Holtz and 23 points a game the year before that.
So what does this mean?
Probably not much. Lincoln Riley had seven years of OC experience before taking over as OU's coach, while Day has about five years OC experience. Riley had a lot of success under an unsuccessful coach (McNeill), while Day has enjoyed success under great to good coaches (Meyer and Addazio).
Outside of his OSU years, Day's offenses didn't exactly light it up. This could be because Addazio wanted to pound the rock and rely on Don Brown’s defense (sound familiar), or it could be that his tweaks to Urban's offense helped net them an extra touchdown a game and almost bring them back to the Tom Herman levels. His impact on Temple wasn't much although a very small sample size, and they did perform better without him than with him. At BC, they seemed to slip pretty bad when he left, but Scot Loeffler took about one year to get them back to the Day level offenses. At OSU, he crushed it, but he was tweaking an offense of one of the best offensive coaches over the last fifteen years. He added 2 points of output from Warinner’s last year as OC, and another 2 points the following year, but never eclipsed Herman’s numbers when he was OSU’s OC.
Riley, on the other hand has always been pretty impactful. Outside of 2011 and 2012, all of his offenses averaged more than 35 points a game. Moreover, he did this with Ruffin McNeill as his head coach, who was only a head coach one more year after Riley left, only to never get another head coaching position. ECU lost about 11 points a game after his departure. At OU, he took them from a team that put up about 35 or 36 points a game, to an offense that never averaged less than 43 points a game and that put up nearly 50 a game this year.
While Day may very well be a very good head coach, I don't think it's fair to put him in the same category as Riley. Riley has always had better offenses, even when working for a mediocre head coach. Day hasn't really had prolific offenses, except when he worked with Meyer, who already had prolific offenses.