WHAT IT MEANS: Johan Santana could not protect a four-run lead and the Mets dropped the ball in the eighth inning -- literally -- en route to 5-4 loss against Pittsburgh in Monday’s series opener.

Santana served up a game-tying two-run homer to No. 8 hitter Michael McKenry in seventh inning.

In the eighth, after each pinch-hitting in the top half, Kirk Nieuwenhuis and Mike Baxter headed to the outfield. They miscommunicated on Neil Walker’s leadoff fly ball to left-center and Nieuwenhuis dropped it for a three-base error. Walker then scored the decisive run on a sacrifice fly by Clint Barmes against Jon Rauch narrowly ahead of a throw from right fielder Lucas Duda.

It was the second time this season Nieuwenhuis had a high-profile drop while manning center field. Against the Giants on April 21, Nieuwenhuis overran a ball to prolong the game, although the Mets salvaged that one, 5-4, on San Francisco miscues.

NOT WRIGHT: David Wright went 2-for-4 with a walk and RBI to lift his average to .415, but he also committed his second and third errors this season.

Wright’s RBI single had lifted the Mets to a 4-0 lead against Pirates starter Erik Bedard.

Neither error proved costly. With the bases loaded in the fourth inning shortly after Wright’s throwing miscue to first base, Santana coaxed a 6-4-3 DP to preserve a two-run lead. In the seventh, after Santana served up the game-tying two-run homer and a walk, Wright misfielded a grounder that advanced the potential go-ahead run into scoring position. But Bobby Parnell struck out Andrew McCutchen and Tim Byrdak fanned Pedro Alvarez to keep the score tied.

LEADING MAN? Andres Torres went 0-for-4 with a walk. He twice struck out and also popped out on a bunt. Torres is now hitless in 14 at-bats and 2-for-38 in his last 11 games. His eighth-inning fielder’s choice stranded the go-ahead run at third in the eighth.

OUT: Scott Hairston was pulled for the bottom of the sixth inning, with Vinny Rottino moving from first base to left field and Ike Davis entering the game.

It was not immediately known if that was a strategic move or Hairston needed to be removed.

Davis and Lucas Duda consecutively were retired in the top of the seventh inning with two runners aboard while facing lefty reliever Tony Watson as the Mets failed to build on a 4-2 lead at the time.

AHOY: Ronny Cedeno returned to Pittsburgh, where he had been the starting shortstop last season and hit .249. He went 2-for-4.

WHAT’S NEXT: R.A. Dickey (5-1, 3.76 ERA) opposes right-hander James McDonald (3-2, 2.68) on Tuesday at 7:05 p.m.