The Baggies registered a hard-earned win against the Saints at the Hawthorns thanks to Saido Berahino's winner.
But West Brom shot stopper Foster received a second-half booking for time wasting and Southampton boss Ronald Koeman was far from happy with Tony Pulis's side's actions when speaking after the game.
As such, opposite number Forster contacted the 31-year-old asking him how he planned to waste time the day after the match.
And Foster duly took to Twitter with the joke...
Time wasting aside, West Brom's win is a huge one in terms of their season. The Baggies are now up to 13th in the Premier League thanks to their three-point gain and face a huge week ahead, with two trips to Aston Villa upcoming - one in the league and one in the quarter-finals of the FA Cup.
For Southampton, meanwhile, the West Brom loss will be a bitter blow to their top-four hopes. All is not lost, however, and the Saints will still have enjoyed a terrific season should they fail to qualify for the Champions League.