Johnson had 20 points, a career-high 17 rebounds and seven blocks, leading No. 8 Purdue to a 67-47 win over Michigan State on Sunday.
"We're really playing at a high level right now," he said.
The Boilermakers (24-5, 13-3 Big Ten) have won six straight to surge into contention for the conference title.
"We're still in the hunt," Johnson said. "We need a little help, but it can be done."
The Boilermakers took control of their latest win with a 16-5 run midway through the first half, then turned the road game into a rout with a 19-4 spurt in the second half.
Purdue won both regular-season matchups against the Spartans for the first time since 1997, and handed them their worst home loss since Duke's 22-point win at the Breslin Center on Dec. 3, 2003.
"We need to move on, but we need to learn from getting punched in the mouth and not responding well," Michigan State coach Tom Izzo said. "The physical nature of that team, I would've really enjoyed it if I wasn't coaching against it."
The Spartans (16-12, 8-8) had won two straight and three of four to improve their shot at playing in a 14th straight NCAA tournament.
"I must say, I'm disappointed, but its been a recurring statement this year," Izzo said. "Give Purdue credit. They played with a tremendous amount of toughness and we just didn't match it."
Michigan State can't afford to fare poorly this week at home against last-place Iowa on Wednesday and on the road against rival Michigan before the Big Ten tournament.
"It's a big game for us on Wednesday," Izzo said.
Kalin Lucas scored 15 of his 23 points in the first half for the Spartans and didn't get much help.
Lucas limped off the court at halftime, favoring his right leg after Izzo said he "tweaked his ankle", but played another 18 minutes in the second. He more than tripled the scoring of any of his teammates.
The 6-foot-10, 221-pound center had plenty of help, but he was quite impressive himself.
Johnson had 13 points, 11 rebounds and two blocks in the first half alone as Purdue led 33-24. He finished 8 of 13 from the field on an array on low-post moves, fadeaway jumpers and made both of the 3-pointers he attempted.
"JaJuan Johnson is playing as well as anybody in the country," Izzo said.
Johnson's defense inside was one of the reasons Michigan State connected on just one-third of its shots.
"I always tell him you have to prove it on the road and he did that," Purdue coach Matt Painter said.