Chivu, 31, spent four years with Ajax before leaving to join Roma in 2003, and his future at Inter is far from certain with his contract due to expire at the end of the season.
De Boer accepts that Belgium international Jan Vertonghen - who attracted the interest of Manchester City during the summer - may soon move on, and he believes Chivu could be the man to replace him.
"He is certainly a serious option," De Boer told De Telegraaf. "Last summer, we looked at Chivu as a possible replacement for Jan Vertonghen, who was at that time subject to a lot of interest.
"Joris Mathijsen was also in our sights, but he decided to join Malaga. After that our choice was Cristian."
Asked if Ajax would revive their interest in January, he replied: "That again depends on Vertonghen, since I don't think Chivu would be happy to be on the bench.
"He's a plan B - a good alternative with great experience at the highest level and a great professional who has always followed Ajax. We've never forgotten about him."
Chivu's agent, Victor Becali, has since played down the suggestion his client could return to Amsterdam.
"There has never been any contact with Ajax," he told TMW. "As is stands we have still not been offered anything in terms of a contract extension by Inter. However, I can confirm that Cristian will not be going to Ajax in January. Only a massive offer could see Chivu leave the club before his contract expires."