Johnson told BBC Radio Cambridgeshire: "I know and everybody knows what we need to improve on - we've got to defend better as a team.
"I would think we're going to need to bring in a player or two and we have been looking and we will keep looking."
Johnson has added to his attack over the past week, with youngsters Jon Obika and Kieran Agard arriving on loan from Tottenham and Everton respectively.
He is now set to turn his attention to strengthening a defence, which has conceded more goals than any other in League One this season.
Johnson was well aware what cost his side in Saturday's heavy FA Cup defeat.
"It's in our own half where we got opened up by the power, pace and cleverness of our opponents," he added.
"But you're not going to find too many League One sides with a forward line like they have at Fulham."
Winger George Boyd missed the game with an ankle problem, but is expected to be fit for Tuesday's league match with Brentford.
Source: BBC Sport
Source: BBC Sport