United battled to an encouraging 1-1 draw with Notts County last midweek as they ramped up pre-season.
And it left Cox encouraged with what he has seen so far.
“We have quite a new group and I am heartened by the gelling and understanding of each other,” he said. “It's ahead of where I thought it would be.
“I'm just happy with where we are. At the minute there are things we are working on and can improve on, but the lads have worked really hard and, at this stage, are in good shape.
“The lads have done really well against some fantastic opposition.
“Fingers crossed we stay clear of injuries and can hit the season with a fully fit squad.”
“It will be a tough test and a good game,” added Cox. “It is going to ask questions of us.
"We have played Notts County and Chesterfield are in that bracket and are a team I think can win the league this season.
“If we are off our game it will ask questions, but that is what we want.
“We don't want to play teams where it’s a testimonial game. I want high tempo and questions being asked.
“Games like this are good because you are asked questions and can sort the problems out.
“Some teams go through pre-season winning every game and then it hits you in the season because you've not been tested.
“Chesterfield are likely to have a lot of possession and will ask questions of our shape and in a defensive scenario.
“As long as we go to the last week knowing the boys have had the right amount of minutes and conditioning then that is all we can ask for.”
Boston fans can also most likely expert further signings in the coming week, with Cox revealing he has targets in mind.
“We are still looking to move the squad and the days going into the 6th August (opening day) wll be key,” he added.
“We can always improve and we are always looking. I’m sure there will be an addition or two to the squad."