PITTSBURGH — Rookie Dalvin Cook has been a second-half back for the Vikings.

In his debut, Cook had 105 his 127 yards in the second half. That turned out fine since Minnesota beat New Orleans 29-19.

On Sunday at Heinz Field, the running back had 61 of his 64 yards after intermission. That didn’t work out as well since the Vikings lost 26-9 to Pittsburgh.

Coach Mike Zimmer said the Vikings “made some adjustments’’ after Cook had just three yards on six carries in the first half.

“We just had to go out there and execute the plays that were being called,’’ Cook said. “I think the second half, we came out with a lot more energy and we just went out and executed the plays that were called and we got a rhythm going.’’

The problem was the Vikings were mostly out of the game in the second half. They trailed 14-3 at halftime and were down 17-3 when Cook finally got going in the third quarter.

Cook had a 25-yard run to the Steelers 1. It was first ruled a touchdown, but a review determined his knee hit the ground just before the goal line.

On the next play, fullback C.J. Ham scored a touchdown on his first NFL carry.

“It was a surreal moment,” Ham said. “You dream about it as a kid. It would have felt better if we would have gotten the win.”

Ham also wouldn’t have minded if Cook had a 26-yard run for his first NFL score.

“I told Dalvin afterward, ‘I’m sorry that I had to take that away from you,”’ Ham said. “He had an unbelievable run, and if he would have scored on that run, it would have been unbelievable.”

The Vikings mostly called Cook’s number on runs. He had 12 carries while reserve running backs Lavatius Murray and Jerick McKinnon had just three apiece.