PHOENIX — Neil Walker, the Mets’ hottest hitter, was scratched from the starting lineup before Tuesday night’s game against the Diamondbacks with stiffness in his lower back. Manager Terry Collins said the second baseman is day to day.

“I want to head this off before it gets too bad,” Collins said, after trainers tried unsuccessfully to loosen Walker’s back.

The Mets could be without Walker for several days, as he was already preparing to miss time to attend the birth of his first child. The due date for Walker’s daughter was Tuesday.

During that span, Walker is hitting .438 with six homers and 14 RBIs with a 1.189 OPS. It is perhaps the reason that Collins has kept Walker in the lineup. Until Tuesday night, he had played in 20 straight games, though the Mets are only 7-13 in that period.

Walker, 30, has dealt with back issues the last several years. He missed four games in June with tightness in his back.

Comeback trail

Yoenis Cespedes went 0-for-3 while playing seven innings in leftfield for Class-A St. Lucie on Tuesday. It was the first game of a doubleheader and Cespedes’ first game action in the field.

advertisement | advertise on newsday

Cespedes, who went on the DL with a strained right quad, hopes to rejoin the team on Thursday though he’s not eligible to be activated until Friday.

“I feel perfect,” Cespedes said in a statement issued through the team. “I’m going to play nine innings in leftfield (Wednesday) for St. Lucie and then hopefully join the team in San Francisco.”

Shortstop Asdrubal Cabrera (left knee) went 1-for-3 with a double while playing four innings at shortstop in the back end of the doubleheader with Class-A St. Lucie.

“I felt really good,” Cabrera said in a statement released through the team. “I felt comfortable running the bases and in the field. I plan on playing seven innings at short tomorrow.”

Niese gets chance

Mets videos

The Mets traded for Jonathon Niese in hopes that he might turn his season around. Now, he’ll get his first chance to stick in the Mets’ rotation, starting with Wednesday night’s series finale against the Diamondbacks.

“He wants to start, bad,” Collins said. “And he knows that this is a huge opportunity for him that he lost in Pittsburgh.”