The 25-year-old has not featured for the Shrimpers this season after he was shipped out on loan to npower Championship outfit Ipswich.
But with an ever-increasing injury list at Roots Hall, manager Paul Sturrock has recalled the versatile player and rewarded him with a start against the Cobblers.
Mohsni told the club’s official website: “First of all I would like to just apologise to everyone at the club for what has happened in the past.
“The manager and I have spoken about returning to the club. The manager has always been very good with me and I have returned to repay the faith he has put in me.
“Hopefully I can play a big part in helping the club after all the injuries they’ve suffered over the last few weeks.”
Sturrock added: “Bilel has returned in a time of need because with all the injuries we are really struggling.
“He and the club regret what has happened in the past. Bilel is a maverick but he is a good man and a very good player and with all the injuries we have he will be a big help.”
St Johnstone manager Tommy Wright has accused Neil McCann of deflection tactics as he denied reporting the Dundee boss to the Scottish Football Association.