PHOENIX — Neil Walker is the Mets’ hottest hitter, so it’s only fitting in this season of injuries for the club that he was scratched from the lineup Tuesday night.

The second baseman had stiffness in his lower back, according to manager Terry Collins, who took no chances: Kelly Johnson, who hit a solo homer in the Mets’ 7-5 win over the Diamondbacks, was shifted to second base, with T.J. Rivera inserted in the lineup against the Diamondbacks to play third base.

Walker, who was hitting .455 over his past 19 games with six homers and 14 RBIs, has battled back stiffness throughout his career, including earlier this season.
“When he had the last one he lost two or three days, so we want to make sure we head this off before it gets too bad,” Collins said.

Walker also remains on standby to return home to Pittsburgh, where his wife Niki is expecting the couple’s first child. The due date for the baby — a girl — was Tuesday.

Jon Niese’s first start since returning to the Mets this month in a trade with the Pirates is scheduled for Wednesday against the Diamondbacks.

The lefty has made four relief appearances for the Mets and pitched to a 10.50 ERA. Recently, he was bothered by knee swelling, but said the inflammation has subsided.

“The conversations you have with Jon, he wants to start bad,” Collins said. “He knows this is a huge opportunity for him that he lost in Pittsburgh and now in a situation where he can take over the job. I totally believe he is going to grab it and run with it. He’s been wanting to do it and now he’s got his chance.”