For the second consecutive week, Northwestern’s offense came out like gangbusters and finished like a busted flush.

The Wildcats, who still haven’t scored in the second half this season, lost to Duke 21-7 on Saturday at Ryan Field despite a solid defensive effort.

Northwestern got away with that approach in their season-opening victory at Purdue, where they scored 31 points in the first half and made them hold up in a 31-27 decision.

In their home opener against the Blue Devils, the Cats (1-1) scored early enough, marching 79 yards with the opening kickoff. But they never got going again.

“I thought we did a great job the first drive,’’ said receiver Flynn Nagel, who had 12 catches for 133 yards. “I thought we were going to put 50 on them, that we were going to keep going down and scoring. It didn’t happen. We’ll fix it.’’

The Devils (2-0) scored three touchdowns in the second quarter, all on passes from quarterback Daniel Jones (nine, 52 and seven yards). He completed 16 of 22 passes for 192 yards.

Jones left the game after a hard sack by Joe Gaziano at the end of the third quarter. It was one of two by the junior end, who led the Big Ten with eight sacks last year.

“I landed on him pretty hard,” the 6-4, 275-pound Gaziano said. “I’m a heavy guy. I can’t imagine that feels good. I hope he’s doing all right.”

Meanwhile, Northwestern again used T.J. Green to spell Clayton Thorson, who is on a “pitch count” after suffering a torn anterior cruciate ligament in the bowl game in December.

“I wish I had control over that,” coach Pat Fitzgerald said when asked about sending in Thorson in obvious passing situations. “I’m just told from the medical team when [Thorson] is in and when he’s out. It is what it is.”

Each Cats quarterback threw a momentum-killing interception in the second quarter.

Northwestern also finished the game without both of its starting tackles, Rawshawn Slater and Blake Hance. The extent of their injuries was not immediately determined.

The offense also lost superback (tight end/fullback) Cam Green to a targeting penalty after Thorson’s interception 8:49 before halftime.

“That was kind of the theme of our offense today,” Fitzgerald said. “The next guy needed to step up, and I don’t think that happened. Guys in the O-line, at quarterback, the superback. It got interesting.”

Thorson, who completed 22 of 38 passes for 198 yards, said his in-and-out status didn’t cause the offensive drought.

“I don’t think that had any effect on it,’’ he said. “Our defense did a great job. We have to step up and score some points. Finish drives. Win our one-on-one battles.”

No one denied that the Cats’ offense wasn’t in sync.

“We have a good team. We just didn’t play very well,” Fitzgerald said. “Offensive football, when the timing and rhythm is off, is really ugly. It’s like bad backyard football playing with your buddies. That’s what it looked like at times.”

It was Northwestern’s second straight Week 2 loss to Duke, which clobbered the Cats 41-17 in Durham, North Carolina, a year ago.

Fitzgerald defended his decision to go for it on fourth-and-goal at the 4-yard line early in the third quarter.

“We were somewhat inept offensively,” he said. “We were down two scores [21-7]. If we kick a field goal, we’re still down two scores. Let’s go be aggressive.”

There was nothing wrong with his logic. It was his offense that was flawed.

“Obviously, we’re disappointed in the outcome,” Fitzgerald said. “We have a lot of work to do. And it starts up front.”

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