He told the club's official website: "Nobody will be more disappointed to see Lee Barnard leave than myself.
"His contribution this season has been massive but he was out of contract at the end of the season and he would have walked for nothing."
But the Shrimpers have agreed to extend the loan of Wolves left-back Scott Malone until the end of the season, and are also hoping to bring in another defender in the near future.
Martin added: "We hope to have a permanent centre-back signed before the weekend. There is also a possibility of taking a striker on also until the end of the season but that might not happen before the Wycombe match."