Right now, it is on track to be his worst.
The 14th-ranked Cornhuskers (4-1, 0-1 Big Ten) go into Saturday night's game against Ohio State (3-2, 0-1) putting up numbers similar to those of the porous 2008 defense and well off those of the dominant 2009-10 units.
Nebraska is allowing 27 points and 377 yards a game. The Huskers have had an opposing running back go over 100 yards in three games and are giving up 152 yards on the ground. They've surrendered 10 touchdown passes, made four interceptions and allowed 224 yards a game through the air.
The same unit Pelini praised for its speed, physicality and leadership has been sporadically aggressive, occasionally confused and reliant on arm tackling.
"It's one thing to do it in practice and another when the bullets are flying," Bo said. "You have to do it. At the end of the day, we were in position. We just didn't make a play. It's not magical."
The defense has an opportunity to get well at home against the Buckeyes, who are struggling on offense and reeling from a series of off-field problems involving several players.
Ohio State center Mike Brewster said he's not fooled by Nebraska's defensive statistics.
"I know they've got some great guys over there," he said. "I know their big thing is the Blackshirts. They take a lot of pride on the defensive side. They've got Jared Crick who is highly touted and (Baker) Steinkuhler who's a really good guy and (Lavonte) David at linebacker. It's going to be a battle and those guys are coming to play, and we've got to be ready."
Ohio State is trying to sort out its quarterback situation and ranks 108th in total offense, averaging 308 yards per game. Its line allowed nine sacks in last week's 10-7 loss to Michigan State, though the Buckeyes get back suspended left tackle Mike Adams for the Huskers.
Suspensions, however, have been extended a game for last year's leading rusher, Daniel Herron, and the top returning receiver, DeVier Posey. The Buckeyes also will be without starting receiver Verlon Reed, who tore a knee ligament against Michigan State.
"It doesn't change anything," Carl Pelini said. "Truthfully, the team we've been breaking down for four weeks didn't have Posey and Herron. We're preparing for the team we're watching on film. If they were playing, it might have made us look more toward last year and see how they were using those guys."
The Huskers have found it difficult to replace secondary members Prince Amukamara, Eric Hagg and Dejon Gomes, all of whom are in the NFL. The Pelinis have started four different combinations in the secondary and have had their lack of depth exposed at linebacker. They're also struggling to get consistent play from the front four.
The coaches are turning up the intensity this week with longer and more physical practices. The No. 1 offensive and defensive units also have been matched against each other more than usual.
Bo Pelini said he won't make drastic changes on defense and said he will keep moving players in and out of the lineup until he finds the right combination.
"You make subtle changes. You don't overhaul," he said. "You keep working to get better. It is about execution, technique and fundamentals. You stay the course."