Matt Harvey walked slowly off the mound, careful not to step on the first-base foul line on the way back to the Mets dugout. A round of boos rose from the stands.

Perhaps they were directed at the Mets ace. Or, more likely, they represented a general sense of displeasure on the way to an 8-5 loss to the Giants. Whatever the reason, the frustration seemed understandable.

At a time when the Mets have enough problems, Harvey looked mortal once more.

For the first time, the righthander surrendered three homers in a game. He also recorded a career-low two strikeouts.

In his last four outings, Harvey (6-4) has posted a 7.20 ERA, including an unsightly 27 hits over 25 innings.

Twice in that span, he has equaled a career high by allowing seven runs, including Wednesday night's six-inning performance that also featured nine hits (five for extra bases).

Harvey's downturn has come at a time when the Mets tore up their plans for a six-man rotation, part of their effort to manage the pitcher's innings in his first season since undergoing Tommy John surgery.

With the loss, the Mets (31-29) dipped a half-game behind the NL East-leading Nationals.

One night after being no-hit for the seventh time in franchise history, the Mets began the day with brighter news, activating catcher Travis d'Arnaud from the disabled list. He had been sidelined since April 20 with a fractured finger and a bone bruise and went 1-for-4 Wednesday night.

"I've had a smile on my face all day today," d'Arnaud said before the game. His promotion was expedited with catcher Kevin Plawecki still battling dizziness.

Manager Terry Collins said reliever Bobby Parnell and infielder Dilson Herrera also should be activated within a day or two, bringing the Mets closer to full strength.

But for at least one more night, the Mets played short-handed, leaving themselves without a backup infielder when they demoted Danny Muno to Triple-A Las Vegas to make room for d'Arnaud.

The Mets still managed to take a 4-2 lead over Giants veteran Tim Hudson.

Wilmer Flores delivered a game-tying two-run single in the first with a soft bouncer up the middle of the infield. Eric Campbell laced a two-out single to push the Mets ahead 3-2 in the fourth.

And in the fifth, Lucas Duda snapped an 0-for-16 slide with an RBI single, giving Harvey a two-run cushion.

The righthander had appeared to settle in after a bumpy first inning, in which Joe Panik lined a two-run shot that put the Giants ahead.

But Harvey quickly unraveled in the sixth, when the Giants seized control of the game with a five-run outburst.

Brandon Belt and Justin Maxwell homered and Buster Posey ripped a two-run double to right on an 0-and--2 fastball. The radar gun flashed 99 mph.

However, velocity provided little cover for Harvey, who was pulled at 100 pitches before the start of the seventh.

One of Harvey's signature traits has been his ability to limit home runs. He gave up a total of 12 in 2012 and 2013 in 2072/3 innings.

But in 792/3 innings of work this season, he already has allowed 12 homers, tied for eighth most in the National League.

Notes & quotes:The final day of baseball's amateur draft took on a local flavor for the Mets, who chose Floral Park's Thomas Hackimer in the 15th round. The righthander went 4-1 with a 1.92 ERA while setting the St. John's record with 15 saves. The Mets also selected local products 2B Vincent Siena (14th round, UConn) and RHP George Thanopoulos (35th round, Columbia).