NEW YORK -- A pair of teams struggling to adapt to new roles will square off Monday night, when the New York Mets host the Arizona Diamondbacks in the opener of a four-game series at Citi Field.

The Mets' Robert Gsellman (5-5, 5.98 ERA) is scheduled to oppose the Diamondbacks' Taijuan Walker (6-7, 3.83 ERA) in a battle of right-handers.

Both teams lost Sunday afternoon. The Mets (54-68) fell for the sixth time in seven games as they lost 6-4 to the visiting Miami Marlins. The visiting Diamondbacks (67-57) lost for the seventh time in 10 games when they endured a 12-5 rout at Minnesota that completed the Twins' four-game sweep.

The skid by the Diamondbacks is imperiling a postseason spot that once seemed ensured. Arizona, which went 69-93 last season and has not reached the playoffs since its most recent winning season in 2011, occupies the second wild-card spot in the National League but is just 2 1/2 games ahead of the Milwaukee Brewers.

The Diamondbacks were the first wild card in the NL at the All-Star break, when they were 9 1/2 games ahead of the then-third place Chicago Cubs. For first-year Arizona manager Torey Lovullo, the task now is to remind his team how it got into playoff position in the first place.

"It wasn't a great series, no matter how you slice it," Lovullo said of the sweep in Minnesota. "We're a good baseball team, and I know that things are working and trending towards that. I just want to make sure they remember how good they are."

The closest the Mets will get to the playoffs this year is in memories of the past two years, when they reached the World Series in 2015 and lost in the NL wild-card game in 2016. New York's expectations this season faded due to injuries and ineffectiveness, and general manager Sandy Alderson has ushered in a youth movement by trading six veterans in the past four weeks.

The Mets understand there will be growing pains like the ones experienced Sunday, when rookie shortstop Amed Rosario was slow to throw the ball to first on what turned out to be an infield single by Miami's Dee Gordon. New York ace right-hander Jacob deGrom threw his hands up in frustration and gave up a three-run homer to Giancarlo Stanton on the next pitch. DeGrom admitted afterward he felt bad about his reaction and said he would apologize to Rosario.

"You talk about the message over here about playing the game right, and today when you're pitching, you've got to pitch," Mets manager Terry Collins said. "And if there's not plays made, you've got to pitch around them once in a while. Same when a guy makes an error -- they don't stop making plays when you hang sliders."

Gsellman will be making his second start since being activated from the disabled list after missing more than six weeks with a let hamstring injury. He didn't factor into the decision Wednesday after allowing three runs (two earned) over 5 1/3 innings as the Mets lost to the New York Yankees 5-3.

Walker will be looking to win for the first time in exactly two months. He lost his second straight start Wednesday, when he allowed five runs (three earned) over five innings as the Diamondbacks fell to the Houston Astros 9-5. Walker is 0-4 with a 4.33 ERA since his most recent win June 21.

Gsellman made his lone career appearance against the Diamondbacks as a reliever on May 17, when he allowed one run in one inning. Walker lost his only start against the Mets on July 23, 2014, when he gave up two runs over five innings as the Seattle Mariners fell to New York 3-2.