The 25-year-old joined Saints on a season-long loan in the summer, having found first-team action hard to come by at Stamford Bridge. Bertrand has been mightily impressive for Ronald Koeman’s high-flying side and now moved for a fee understood to be worth up to £10million, signing a four-and-a-half year deal. Press Association Southampton had also been looking at Feyenoord’s Tonny Vilhena, but a deal failed to materialise. Koeman had not initially planned to bring in anyone other than Werder Bremen’s Eljero Elia, only for mounting injury problems and Jack Cork’s move to Swansea to lead him to act. Saints cashed in on Cork after he rejected a lucrative contract extension, but they managed to hold on to a number of key players. Nathaniel Clyne, Morgan Schneiderlin and Jay Rodriguez were amongst those heavily-linked with a move away, yet they will remain at St Mary’s for the remainder of the campaign. The left-back won the Champions League with Chelsea and has played 27 times in all competitions for top-four contenders Saints this term. Bertrand is joined at St Mary’s by deadline-day acquisition Filip Djuricic, who arrived on loan for the remainder of the campaign earlier in the evening. The 23-year-old attacking midfielder joins Southampton from Portuguese giants Benfica, having ended a loan spell at Mainz in the Bundesliga. Djuricic made 12 appearances in all competitions for the German side before linking up with Serbia team-mate Dusan Tadic, who welcomed the new signing. “He is my good friend and he is a good player,” he said. “He is a number 10 and he knows how to play football. He had a little bit of a difficult period at Mainz, but he is a really good player. “He is 23 years old. At Heerenveen he was really good and then he went to Benfica, where he did not so much. “At Mainz it was similar but he has potential and he has quality.” Southampton have made Ryan Bertrand’s move from Chelsea permanent at the end of what has been a successful transfer window at St Mary’s.