Brandon Thomas-Asante scored the only goal of the game just before the hour mark, as the U’s were beaten 1-0 at Coventry City in the second round.
United head coach Buckingham wanted to see more fluidity from his side when in dangerous areas of the pitch.
He said: “Up until the sloppy goal that we’ve given away, it was a pretty even game. The goal gave them a bit of momentum for 10 or 15 minutes, and Matt Ingram has made two good saves off the back of that.
“We then regained a bit of control back, and we arguably should’ve scored two at the end, with Mark Harris across the box and the other which falls to Will Vaulks on the edge.
“The league has always been our priority, without being too corny with the saying. It’s given us a chance to look at players, and we’ve done that.
“We had Dane Scarlett, who started up front and got 80 minutes, and Owen Dale, who we welcomed back from injury, and we got 60 minutes into him, plus Malcolm Ebiowei and Louie Sibley.
“It gave us a chance to look at those in this game, to add minutes, but also see what they can do potentially for Saturday.
“I thought we got up the pitch pretty well. We had different ways of getting out, and we showed that.
“It was in the final third where we got into good areas. It was just the crispness and the cleanness of the decision-making, or the movement in and around.
“You get that with time and those relationships, and we’ve seen in the three games in the league, when players play together more often, you get that.
“It’s not an excuse for those that play, they know what we’re doing, but at times, that can look a little bit clunky.
“I just felt when we got to the final third, at times, we weren’t as crisp in our decision-making and other bits that we normally are.”
Buckingham though was pleased with the improvement he felt the U’s showed after a triple substitution made just after the hour mark.
“We saw that with the changes we made around 60 minutes. After the goal, that’s when we did start creating,” he said.
“We had a couple of moments in the first half, and we chose to pass instead of shoot. There were two of three occasions.
“I’ve made no secret of it, we’re still in the market for certain players, and we’ve got until Friday for that.”