Albion's next boss will have to rebuild from the back following the departure of player of the season Matt Upson to Premier League newcomers Leicester City.
The 35-year-old former England centre-half has signed a one-year deal with the Foxes which comes into force on July 1, once his existing 12-month contract with the Seagulls has expired.
Albion wanted to re-sign him but the lure of a final fling in the top flight was too good for Upson to turn down.
The Seagulls have already lost goalkeeper Tomasz Kuszczak and on-loan Wolves left-back Stephen Ward from the second-meanest defence in the Championship.
Right-back rivals Bruno and Inigo Calderon are both expected to agree new one-year deals but one of the first priorities for the successor to Oscar Garcia will be to plug the gaps in the centre of defence created by Upson’s move to the East Midlands and Adam El-Abd’s January sale to Bristol City.
Captain Gordon Greer and 22-year-old Lewis Dunk are now the only recognised first team central defenders, backed up by Latvian international development squad prospect Vitalijs Maksimenko, who spent the second half of the season on loan to Kilmarnock.
Upson, who played in the top flight for Arsenal, Birmingham, West Ham and Stoke, said: "I am really delighted. It is a great opportunity and something I am very grateful for.
"I played against them (Leicester) last season and I liked the way the team played and the intensity they had. Obviously it is a chance to play in the Premier League again which was something I have been striving to do since I stepped down into the Championship. From that point of view it is a great opportunity.
"I enjoyed last season and had a great time at Brighton but this was too good an opportunity to turn down."
Albion goalkeeping prospect Christian Walton is part of the England under 19's squad launching their elite round of qualification for July's European Championship against Montenegro at Walsall today (3pm).
Republic of Ireland under 18's goalkeeper Harry Doherty, released by Charlton, has been offered a one-year deal by the Seagulls after a week-long trial.