The Greek international striker left Parkhead after four years this summer, but has yet to agree terms with another team, and the continuing availability of a player capable of making an impact in the Champions League interests the club.
In a curious twist, however, Southampton manager Ronald Koeman is also monitoring Samaras's situation and could yet move to reunite him with goalkeeper Forster, 26, who has signed a four-year deal at St Mary's.
It is the second time in little over a year that the English Premier League outfit have raided Parkhead for one of their top players, after Victor Wanyama joined the south-coast club for £12.5m last July.
Celtic manager Ronny Deila last night fanned speculation over Samaras by declining to quell talk over a possible return for the player who was regarded by the Parkhead fans as a Champions League talisman.
"He is a player we know well," said the Norwegian. "He has good qualities, but it has to be right for everybody. We want to keep our players.
"That is the most important thing, but you never know in football. Hopefully when it comes to September 1 you have a better team than when you started."
At St Mary's, Forster will compete with Artur Boruc - another former Bhoys goalkeeper - for the number one spot.
"Fraser is a very talented player and we are delighted to be able to bring him to Southampton," manager Koeman told the club's official website.
"This is another important part of our rebuilding process ahead of the start of the season."
Forster links up with former Celtic team-mates Wanyama and Jos Hooiveld at Southampton, where he work with England goalkeeping coach Dave Watson.
"The fact that he has worked with Dave Watson before means we already know all about his strengths and weaknesses, and I am confident that we will be able to help him improve even further," Koeman added.
"Fraser is the perfect fit to help complement the players we already have and he will add a lot of quality to the squad.
"He is an exciting English player with a very good reputation within the game and, at 26, he will enjoy the best years of his career at Southampton."
Forster is unlikely to be the last acquisition made by Saints before the end of a transfer window which has seen the core of last season's successful side decimated.
Rickie Lambert, Adam Lallana and Dejan Lovren have moved on to Liverpool, while Calum Chambers exited for Arsenal and teenager Luke Shaw moved to Manchester United.
Talk remains over the futures of Morgan Schneiderlin and Jay Rodriguez with Tottenham interested, but the club have made it clear to both players that they will be staying.
Saints chairman Ralph Krueger said this week that the club was now "building and not in a reducing phase", with Saphir Taider joining from Inter Milan the following day in a season-long loan which saw disruptive club-record signing Dani Osvaldo go the other way.
Taider's move is with a view to a permanent deal, like the recent deal that brought Ryan Bertrand from Chelsea, while Koeman has signed Graziano Pelle from former club Feyenoord and FC Twente's Dusan Tadic.
Saints are now trying to complete deals for Sporting Lisbon's Marcos Rojo and Florin Gardos of Steaua Bucharest, while Koeman confirmed on Saturday that Leeds had rejected their bid in the region of £4.5million right-back Sam Byram.
"We lost Chambers to Arsenal - it (Byram) was a player we had on the list and it's true that we had a bid for him, yes,'' the Dutchman said after the 1-0 friendly loss to Bayer Leverkusen.
"What I heard was that Leeds didn't accept it."