The defender, who is out of contract at Premier League side Wolves, made nine appearances after arriving at London Road on loan back in March.
But Johnson is continuing to plan ahead and revealed he does have other options at his disposal should Little's proposed move not go through.
"Mark is happy to be at the club, but his agent is away on holiday at the moment," he told BBC Radio Cambridgeshire. "I hope it's pretty close now. It's just a case of his representative coming back and we'll be nearly there."
"You have to have other irons in the fire because if you're chasing a quality player you can never be sure you have them until they sign on the dotted line."
On other transfer activity, the former Bristol City chief added: "We've made two offers and they're mulling over the terms at the moment so it should be done soon. We're ideally looking for six or seven players and I'm looking at strengthening in most positions."