The 33-year-old has been loaned to the League Two club for the rest of the season, having made just two starts for Fleetwood.
And after heading away from Highbury, the former Blackpool man revealed his frustration at spending so long on the sidelines, kept out of the first-team picture first by Steven Schumacher and more recently by Jeff Hughes.
Southern said: “I’m champing at the bit to get some football under my belt. It’s been very frustrating.
“I signed in the summer, got injured in pre-season.
“I needed a hernia operation and I missed about four weeks.
“I’ve been fit from about the middle of September but have only played a couple of games.
“The manager feels he can send me out on loan. He’s got players to cover me, or rather play instead of me.
“I want to be playing in games and to be wanted.
“I’m delighted to go to a progressive club who are doing well.”
What’s important now for the experienced midfielder, who didn’t travel with the squad for Fleetwood’s last away game at Leyton Orient, is grasping his chance.
“I’ll have to bide my time and train hard,” he added. “All I want is an opportunity.
“I’ve got to make sure that when I’m needed I do myself justice.”
Last week Southern took to social media to assure Town fans he was fit and available, despite not even being on the bench for almost a month.
Ahead of Saturday’s 2-0 defeat by Oldham Athletic, the former Huddersfield palyer tweeted: “For the countless people asking ‘are you injured?’, the answer is, ‘No, I’m not and I have been fully fit for four months’.”
Southern added the hashtags #traineveryday and #justsaying.
Fleetwood manager Alexander insisted Southern and others not presently in his team, like keeper Scott Davies, were not being forced out of the group.
He said: “When players aren’t in the squad the only problem they have is not being picked.
“They train the same as everybody else They have the same days off.
“That’s the only way I can go about it. I don’t banish anybody.
“I’m not connected with social media. I didn’t know about it (Southern’s tweet).
“There are no problems, but I pick people on performance alone.”
Fleetwood are at home to Crawley Town this weekend, while Shrewsbury host Stevenge.