The Spireites sunk to a 2-0 FA Cup defeat to high-flying League One outfit Bradford City.
Goals from Alex Gilliead and Alex Jones ended the visitors’ FA Cup campaign at the first hurdle.
And Lester admitted: “It’s not a balanced squad and that’s something we’ll have to address in January.
“We’ve got a lot of central midfielders, we know that, and that’s something we’ll have to look at.
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“They’re a lot higher than we are at the moment and that showed.
“That’s where we’re aiming to get to and we’ll get better with time.
“They were five minutes away from the Championship and they’ll be close to that again.”
Both goals were avoidable according to Lester, with his defenders failing to track runs.
It’s something which has plagued the Spireites all season, but they are working hard to improve on the training ground.
And Lester added: “We started the game okay, but we gave away a bit of a sloppy goal.
“We need to be more aggressive with our runs in front of their back four.
“It was two goals that we shouldn’t have conceded.
“We’ve got Swindon next and we’ll head there on the front foot.
“They’re doing well, but they’ll be no better than Bradford and then we’ve got two home games so a decent run of fixtures coming up.”
It is the first time the Spireites have been knocked out in the first round of the competition since the 2011/12 season.
Despite naming a fairly strong side for the trip to Yorkshire, Lester would much prefer a lack of distractions as his team try to fight their way out of the League Two drop zone.
Retaining league football must be the highest priority for the struggling Spireites, although Barnsley showed just what a cup run can inspire after going from the relegation zone to promotion through the League One play-offs in the 2015/16 campaign as they won the Johnstone’s Paint Trophy.
Bradford: Doyle, Hendrie, Kilgallon, Thompson, Knight-Percival, Gilliead (Robinson, 87), Vincent, Dieng (Devine, 78), Taylor, Wyke, Jones (Law, 71). Unused subs: Raeder, McCartan, Patrick, Poleon.
Chesterfield: Anyon, Barry, Evatt, Hird, Wiseman, Kellett, McCourt, Reed (O’Grady, 71), Dimaio (Sinnott, 78), Rowley (Mitchell, 84), Dennis. Unused subs: Jones, Maguire, Weir, De Girolamo.
Referee: Anthony Blackhouse (Paul Marsden, 32).