Grimsby manager Paul Hurst believes the 2-0 Sky Bet League Two defeat to Crewe should act as a "reality-check" for his recently-promoted side.
The Mariners went into the game on a high after last weekend's victory at Luton but were brought back down to earth with a bump as the Railwaymen steamed away from Blundell Park with all three points.
George Cooper grabbed the opening goal of the game on the stroke of half-time after pouncing on a mistake from the hosts before James Jones added a second with five minutes to go.
Alex striker Chris Dagnall had earlier seen a first-half penalty saved by James McKeown after Josh Gowling handled in the box, while Grimsby striker Shaun Tuton had a goal chalked off for offside just before the break.
Boss Hurst said: "For the first time this season, there's a disappointment in terms of the performance.
"I have to say, we were against a team that's only lost one game, were a League One outfit last season and there haven't been mass changes there.
"You have to give the opposition credit, they stopped us playing and did well.
"It was terrible for us to concede just before half-time and, in the second half, while a lot of the play was in their half, we didn't create or ask too many questions of them.
"We were quite a way off our own standards - especially after the performance of last week.
"To drop off so quickly after that was disappointing. It was our worst performance to date and definitely a reality-check - that's exactly what was said in the dressing room at full-time."
Crewe manager Steve Davis, who saw his side move into the top three with victory, said: "It's a far contrast from the past couple of seasons, when we have struggled.
"We have restored some pride, work ethic and good performances. It's the start of a long season, and we are pleased with how we have started.
"We have to keep it going, but we won't get carried away - I know what football can be like. I know what can happen if complacency sets in, but that will never happen with me. We will always work hard behind the scenes.
"We are delighted with the position we are in. It's a reward for the players, for their hard-work and grafting in the summer. Hopefully that will continue."