On Monday, Justin Simon was voted in as the Big East Defensive Player of the Year. Two nights later, he proved the league’s coaches made the right decision.

The 6-foot-5 wing keyed St. John’s NCAA Tournament-clinching 82-74 win over No. 10 DePaul in the outbracket round of the Big East Tournament, scoring 18 points and locking down Blue Demons leading scorer Max Strus.

“This is what you play for — you play for moments like this,” Simon said after seventh-seeded St. John’s snapped a three-game losing streak to move into a Thursday night quarterback showdown with No. 2 Marquette at the Garden. “I just go out there with a mindset to defend whoever my assignment is. I’m always ready to accept a challenge. Coming into the year, coach [Chris Mullin] told me he wanted me to be a great defender all across the board. I feel I’m doing that.”

Just 10 days ago, Strus started the St. John’s (21-11) regular season tailspin, pouring in 43 points and six 3-pointers. He managed just 14 on Wednesday on 4-of-12 shooting, and six of those came in one minute when the result was no longer in doubt.

“That’s why he’s the Defensive Player of the Year,” St. John’s coach Chris Mullin said.

The coaching staff also had a game plan ready to combat DePaul and Strus. It played off shooting-challenged point guard Devin Gage, freeing up Shamorie Ponds to help on the Blue Demons’ other key players. While Gage scored 19 points, he made just 9-of-20 shots. Strus and forwards Paul Reed and Femi Olujobi struggled with fewer touches.

“Our game plan really was the biggest factor,” senior Marvin Clark II said.

“It definitely threw them off a little bit,” added Simon. who also had six rebounds, four steals, two assists and two blocks.

Simon, however, was the key. In the last game against DePaul, he played only 27 minutes due to foul trouble as Strus got loose after halftime. This time, he logged 38 minutes, rarely leaving the floor.

“That was the biggest thing,” Simon said.