Bruce blames his balky back on a lack of stretching

Jay Bruce’s lower back was sore when he arrived at Citi Field on Friday and only worsened as the day progressed.

By the fourth inning, he was ready to call it a night, knocked out by back spasms following his second at-bat against Cubs’ right-hander Tyler Chatwood.

“It just wouldn’t loosen up on me,” Bruce said following the Mets’ 7-4 loss. “I really don’t anticipate it being serious.”

Manager Mickey Callaway tentatively has Bruce scheduled to play Saturday, but the outfielder will be evaluated when he arrives.

Bruce had started at first base Thursday — it was only his second start of the season at the position — and indicated his back spasms may have resulted from not stretching properly.

Noah Syndergaard’s next turn in the Mets’ rotation is unlikely to come before the Subway Series.

With Syndergaard on the disabled list after straining a ligament in his right index finger, team officials were in the process Friday of finalizing a rotation through the next week-plus — with Syndergaard unlikely to be ready for Tuesday, and either Seth Lugo or Jason Vargas taking his turn against the Orioles.

It is also unlikely Syndergaard would pitch Wednesday, because that would eliminate him from consideration for next weekend’s Subway Series against the Yankees at Citi Field.

Zack Wheeler may pitch against the Orioles on Wednesday, though Callaway also could choose between whomever doesn’t start Tuesday, Lugo or Vargas, for that turn in the rotation.

The Mets are confident Steven Matz, who avoided the DL after he strained his middle finger swinging the bat, will be in position to start Sunday’s series finale against the Cubs.