After the superb win at Carrow Road on Saturday, a strong Hatters side struggled defensively against a far more direct Mariners, confirming to the boss he need to bring in an experienced central defender.
He said: “I learned so much and I’m glad I fielded the team I did.
“I kept certain areas the same, because the FA Cup and playing against Norwich and Wolves takes care of itself in terms of players being up for it and the challenge in front of them.
“When you go to someone like Grimsby, it’s a totally different game. It was aerial and we came unstuck.
“They scored to get them up and running, it was a poor free kick to give away from Jan (Janos Kovacs) and its just gone straight in.
“We need to get a head on it and can Brilly (Dean Brill) see it? I’m not sure. It’s a bit of a fluke. For the second, JJ (O’Donnell’s) tried to block it and its gone under his studs, then we got caught on the counter after that.”
Buckle did feel his side were deined a spot kick though when Stuart Fleetwood tumbled in the area during the first half, as he said: “I thought it was a blatant penalty on Fleets and to get that changes the game.
“We had some really good spells in the game, our passing was excellent at times, but we just couldn’t get that goal.”
Although Hatters missed out on a two-legged semi-final with either Dartford or FC Halifax, Buckle saw the advantages of missing out further fixture congestion.
He said: “Something had to give sooner than later and I’d rather that be the Trophy than the League or the FA Cup.
“I’ve been asked if I would swap the heroics of the FA Cup to get promoted and I always say yes.
“That doesn’t mean we set out to lose because we didn’t. I’m not overly disappointed that we’re out though.
“If you’d asked me at the start of the season, if you win your three games in hand you go top and you’re in the fifth round of the FA Cup, I’d have taken that.”
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