Boston United defence have 'still got a lot to learn' despite FA Cup victory - Craig Elliott
Goals from Terry Hawkridge, Jordan Burrow, Luke Shiels and Fraser Preston ensured the Pilgrims will be in the hat for Monday's draw.
But it was the Bulls' goals from Lynton Karkach and Tom Slone which had Elliott asking questions - even if he is willing to offer his squad concessions as they continue to grow together.
"I said to the players I have to meet them half way as there's a lot who haven't played competitive football for six to eight months. Gibbo (Lewis Gibbens) has hardly had anything in a year," he said.
"But we need to improve defensively moving forward. It was quite a young back four today so I think you could see that at times, still got a lot to learn.
"We've not worked with the same back four in pre-season, it was all about giving people minutes.
"Now we need to get that spine to us and get people working, but it'll come."
Scott Garner was rested due to a rib injury while Luke Shiels continues to play through the pain barrier with an injury the club are still trying to pinpoint.
"I'm trying manage Luke Shiels best we can," Elliott added.
"It might be a bit of cartilage, we're not sure. He's getting through games but aching after matches. He's finding it difficult and it's a bit of a worry for me.
"Hopefully Scott Garner will be back for Tuesday (at Leamington) I hope."
While Elliott has concerns about his defence, he's more than happy with how his side have looked in attack.
He said: "It's important we get the balance right. We had a lot of options and creativity going forward. I thought we were very good.
"We need to get the balance right as we'll come across some very good teams in out league.
"We need to be gritty and hard to play against and have that bit of quality."
Andi Thanoj went close in the opening moments as his 20-yard free kick rattled the Mansfield crossbar, but two goals in as many minutes settled any early Pilgrims nerves as Hawkridge and then Burrow found the net.
With 12 minutes gone indecision between Bulls keeper Hugo Warhurst and defender Jack Harrison allowed Hawkridge to nip in and slot the ball home into an unguarded net.
That lead was doubled seconds later as Burrow was gifted a simple finish after Warhurst's save to deny Jordan Thewlis landed invitingly in his path.
Boston looked to be cruising and summer signing Connor Dimaio, in particular, was having fun with slick passing and an eagerness to get things moving in the final third.
But as Mansfield settled into the contest they began to show Boston that the three-division gap meant nothing to them.
Karkach pulled one back with a neat volley after he lofted the ball over United defender Lewis Gibbens to allow him a sight of goal.
And it could have been all level had Ross Duggan's fierce strike been millimetres to the side, but his effort rebounded into play off the post.
Thewlis' deflected effort struck the post at the other end before Shiels restored United's two-goal advantage.
Making the most of sloppy defending he headed home a corner unmarked from close range, Thanoj chipping in with his third assist of the half.
The second hal didn't match the first for a lightning start, but Dimaio took his turn to rattle a post with a 20-yarder before, from the resultant corner, Shiels' header was well saved by Warhurst, pushing away the goalbound effort.
United debutant Preston had only been on the pitch four minutes before his low drive made it four and put the tie beyond doubt, seconds later Thewlis' curler drifting inches wide.
Back came Mansfield, Ross Fitzsimons tipping Jordan Annable's header over then denying Harrison on the spin.
Substitute Slone go his name on the scoresheet in the 85th minute as he powerfully headed home Ben Algar's cross.
But despite some spirited attempts from the Bulls, Boston booked their place in the next round.
UNITED: Fitzsimons, Platt, Shiels, Thanoj (Preston 62), Burrow (Archer 75), Thewlis, Gibbens, Warren, Askew, Dimaio (Green 70), Hawkridge; Subs (not used): Tootle, Duxbury, Garner, Crook.
MANSFIELD: Warhurst, Marsden, Wilde, D'Laryea, Annable, Harrison, Karkach (Walker 75), James, Duggan (Slone 82), Sykes (Buxton 67), Algar; Subs (not used): Stancliffe.
Ref: Oliver Makey.
MORE PILGRIMS: United v Bulls - as it happened