Johnson told BBC Radio Cambridgeshire: "We have defensive targets in mind to bring in, but it's just a case of waiting to see what happens.
"We can't have someone offering peanuts for one our best players."
Morgan has been a key player for Posh in recent seasons, but is believed to be interested in linking back up with former boss Darren Ferguson at Preston.
The Lilywhites are currently awaiting the completion of a takeover move after which Ferguson's summer spending can begin.
Morgan came close to leaving London Road for Notts County in January, but the deal fell through after Posh suffered an injury crisis.
Johnson has brought in four new faces since the end of the season with Grant McCann, Dave Hibbert, James Wesolowski and Mark Little all coming on board.
He has said he would be happy if the new season were to start tomorrow, particularly given the likes of Joe Lewis and Craig Mackail-Smith remain at the club.
George Boyd is the player most likely to move on given his contract expires in the summer of 2011.
One final offer of new terms has been made, but Johnson has said they can only wait and see what happens next.
"We would be in a great position if we could keep Boyd as we'd have four players with proven goal scoring ability," he said.
"As it stands at the we've got a strong squad for League One, but you can't taken anything for granted."
Source: BBC Sport
Source: BBC Sport