According to reports, Adam Lallana has already put pen to paper completing a £25million switch to Liverpool which will be officially announced in the coming days.
If true, the news comes as a further blow to Southampton fans following the £4million sale of Rickie Lambert - also to Liverpool - earlier this month and the £30million sale of Luke Shaw to Manchester United.
Lallana made 265 appearances for the Saints and scored 60 times after breaking into the first team from the academy in 2006. The outgoing Saints captain leaves big boots to fill at St Mary's, but with Koeman holding a reported £70million transfer kitty, the Dutchman certainly has the means to do it.
Here are three possible candidates:
Tadic is a player Koeman will be extremely familiar with, having watched the Serbian closely in Eredivisie while managing Feyenoord. The 25-year-old attacking midfielder has established himself as one of the top players in Holland since making a €7.7million switch to Twente in 2012 and netted 16 times last season, making 14 assists.
The Serbian international has already been reported as a possible replacement for Lallana and is thought to be available for around £10million. Such a deal could prove a steal for the Saints, leaving plenty of money left over to bolster other areas of the squad.
The Portuguese youngster has attracted attention from all over Europe lately, and Southampton have been included in a list of possible suitors. The 21-year-old Braga midfielder is adept at playing through the middle or on either wing, making him positionally an ideal replacement for Lallana.
Silva's pace, direct style of play and eye for goal have led to him being touted as the Portugeuse answer to Eden Hazard and the youngster was included in Paulo Bento's 23-man World Cup squad - although he failed to make an appearance.
After signing a five-year contract extension at Braga last summer and with further reported interest from Roma and Monaco, it could take a large bid from Southampton to land the midfielder.
The Dutchman was Koeman's number 10 at Feyenoord and his industrious work-rate and goal-scoring potential could make him an ample replacement for Lallana. With such an overhaul needed to replace Southampton's outgoing key men, it would make sense for Koeman to recruit players already familiar with one another.
Southampton are reported to be close on sealing a deal to sign Immers' Feyenoord team-mate Graziano Pelle to replace Rickie Lambert up front, so do not be surprised if the 28-year-old midfielder is next on the shopping list for the Saints boss.