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WHAT IT MEANS: Collin McHugh tossed seven scoreless innings in his major league debut, but Jordany Valdespin's subsequent misplay in center field proved costly as the Mets lost to Colorado, 1-0, Thursday afternoon.

The Rockies completed a four-game sweep at Citi Field for the second straight season and dropped the Mets 11 games under .500 for the first time since the 2009 season.

The Mets (57-68) tumbled into fourth place for the first time since June 6 -- a half-game behind Philadelphia, which plays tonight against Cincinnati. They are only a half-game ahead of NL East cellar-dweller Miami, which is idle today.

In a scoreless game in the eighth, Tyler Colvin sent a routine line drive to center field off Bobby Parnell. An inexperienced outfielder, Valdespin initially broke in on the ball, and it landed over his head, officially for a triple. Chris Nelson followed with an RBI single.

Valdespin was making his fifth career major league start in center field. He has appeared in 23 professional games at the position, all this year, between the majors and Triple-A Buffalo.

McHugh, an 18th-round pick in 2008 out of Berry College near Rome, Ga., limited the Rockies to two hits and a walk -- which was semi-intentional to Carlos Gonzalez -- while striking out nine in a 100-pitch effort.

McHugh surrendered a game-opening opposite-field double to Charlie Blackmon, but stranded him at third base by striking out Jordan Pacheco looking at a 91 mph fastball and Gonzalez swinging at a 92 mph fastball to end the top of the first.

The nine strikeouts were the second most in franchise history in a major league debut, trailing only Matt Harvey's 11 in Arizona on July 26.

McHugh became only the third pitcher in franchise history to toss seven-plus scoreless innings in his major league debut. The others: Dick Rusteck (nine scoreless vs. Cincinnati in 1966) and Masato Yoshii (seven scoreless vs. Pittsburgh in 1998).

BLANK SLATE: Tyler Chatwood, Brooklyn-native Adam Ottavino, Rex Brothers, Will Harris and Matt Belisle combined to blank the Mets. The Mets went 0-for-10 with runners in scoring position and left 12 on base.

WHAT'S NEXT: The lowly Astros visit Citi Field this weekend. Jon Niese (10-6, 3.82 ERA) opposes right-hander Jordan Lyles (2-10, 5.70) in Friday's series opener, on Merengue Night at Citi Field.