"We'll keep looking, I'd like another defender in the club at the moment," Johnson told BBC Radio Cambridgeshire.
"It's taking Gabby a long time to get sorted out and we're not sure how long that will last."
Zakuani, 24, has had his pre-season hampered by a persistent groin problem, while the departure of Craig Morgan to Preston and Queudrue's decision to choose Greece over Posh has left Johnson short of central-defensive options.
Speaking after his side's 5-1 friendly loss to Leicester, he said: "I thought up until today Kelvin Langmead and Ryan Bennett had done very well as a pairing.
Better to get the bad performances and mistakes out of the way in games against teams like Leicester than when it really matters
oldposhpete
"And Gabby's running out of time to get a place in the squad ahead of the Bristol Rovers game on the first day of the season."
The defeat by the Foxes put a dampener on what had been a progressive pre-season so far and Johnson made it clear the performance fell short of his expectations.
"All days off are cancelled," he said.
"Tomorrow we'll be in training, because if you fall off that horse you've got to get back on it straight away.
"They're in early in the morning and we'll have another little chat. They know how I feel and they know they've got to do a lot more."
Meanwhile Johnson confirmed that striker Craig Mackail-Smith and full-back Mark Little had been rested to give them a better chance of being ready for the League One kick-off.
Winger Chris Whelpdale will step up his race for fitness in a friendly at Histon on Wednesday night.
Source: BBC Sport
Source: BBC Sport