While injuries have struck hard, sidelining Noah Syndergaard, Steven Matz and Seth Lugo, the lack of results from the likes of Gsellman, Matt Harvey and Zack Wheeler has manager Terry Collins puzzled.
"It has been shocking to me. Shocking," Collins said. "Even though we lost Matz, we lost Lugo and Noah went down, we still had what we thought was very good pitching here.
"We're having a tough time getting through the fifth inning. You are going to struggle in this league if your starting pitching can't get you through. You are going to have nights where they are off and you have to get them out early, but we should be pitching better than we are."
General manager Sandy Alderson said Friday it would be foolish to expect a lot out of the rotation in its current form, which includes fill-ins Tommy Milone, Adam Wilk and Rafael Montero.
But the continued struggles of Gsellman and Harvey leave the Mets with question marks where they thought they had answers.
Following an ineffective outing by Harvey on Friday, Gsellman walked a fine line to escape a bases-loaded jam in the fourth, but it was only a precursor of what was to come.
With the Mets up 4-2, Gsellman allowed three hits and a walk to start the fifth inning before being pulled. Hansel Robles proceeded to allow four more hits in the frame as the Brewers scored eight runs to grab control of the game.
"I'm just up in the zone," Gsellman said. "I'm getting balls up that they can hit and get out of the infield. I'm rushing and being too quick and not keeping my head still. I'm putting the ball right down the middle."
Gsellman showed promise late in 2016 when he made seven crucial starts for the Mets down the stretch. He had a 2.63 ERA in those seven starts and allowed more than two earned runs just twice.
This year has been a different story; the 23-year-old has been mostly ineffective aside from an April 19 start in Philadelphia. He has a 9.00 ERA over his past four starts and has failed to work past the fifth inning in each of those outings.
"I saw this kid pitch in crunch time last September as good as anybody we've had," Collins said. "And we've had some good ones come up. Right now, we're just stumped about what is going on. I just have to rely on my coaches to continue to work and get it figured out."
To Collins, it is hard to believe the starting rotation, considered one of the best and deepest in all of baseball entering the season, is the major concern with his club at this point. Until Matz and Lugo are able to provide alternate options, Harvey, Gsellman and Wheeler need to step up.
"They came in highly, highly regarded and probably have put too much pressure on themselves instead of just pitching," Collins said. "They know a lot is expected, and therefore they heaped all that self-pressure. You just have to relax and go pitch your game to make quality pitches, and we're not making them."
The only reliable member of the rotation thus far has been Jacob deGrom, who the Mets are hoping will provide a strong start Sunday.
"Every staff has to have that guy," Collins said. "They have to have that guy they can turn to that stops losing streaks. Even if he doesn't win the game, he keeps you in the game to where it allows you a legitimate chance to win a game. Tomorrow we need him to step up desperately."
Andrew Gruman is a contributor to MLB.com based in Milwaukee and covered the Mets on Saturday.