Ryan Bertrand has blossomed since swapping Chelsea for Southampton

​​​​​​​Ryan Bertrand returns to his old stomping ground when Southampton face Chelsea live on Sky Sports on Tuesday night. We examine how the full-back has blossomed on the south coast.

Marcos Alonso's late withdrawal from Chelsea's recent defeat to Manchester United left Antonio Conte without a natural replacement at left wing-back. As he pondered his options at Old Trafford, he could have been forgiven for cursing the man in the opposite dugout.

It's a heavyweight week of action this week on Sky Sports with three huge Premier League games It's a heavyweight week of action this week on Sky Sports with three huge Premier League games

Back in January 2015, it was Jose Mourinho who sanctioned Ryan Bertrand's permanent move from Chelsea to Southampton. The academy product had been seen as a long-term replacement for Ashley Cole and started in the 2012 Champions League final, but he left Stamford Bridge having made just three appearances under Mourinho.

The 27-year-old does not rank as highly as Romelu Lukaku and Kevin De Bruyne on the list of ones who got away during Mourinho's time in charge, but Chelsea could use him now. In fact, the Evening Standard recently reported that Conte is keen to bring him back to the club this summer.

The full-back in action against Crystal Palace earlier this season

It would be no great surprise. Bertrand has gone from strength to strength in three seasons at Southampton, becoming a key player and an established England international. He has predominantly shone in his natural full-back position, but he even impressed on the left of a back three under Ronald Koeman. For Conte, that versatility is an added attraction.

In a defensive sense, Bertrand made his biggest strides under Koeman. Southampton finished the 2014/15 and 2015/16 seasons with the second and fifth-best defensive records in the Premier League, with Bertrand only missing 10 games out of 76 over the course of the two campaigns.

Bertrand found consistency with the regular football he was never afforded at Chelsea. He was named in the PFA Team of the Year in his first season on the south coast. Since then, both Koeman and his successor Claude Puel have allowed him to express himself, too.

Bertrand has created 15 big chances in the last three seasons

His overlapping runs have become a feature of Southampton's style, and few defenders have been so effective in the attacking third. Bertrand has contributed more assists (nine) than full-backs Kyle Walker, Antonio Valencia and Hector Bellerin in the last three seasons. West Ham's Aaron Cresswell is the only defender to have provided more big chances.

What is a big chance? Opta define a big chance is a situation in which a player should reasonably be expected to score, such as a one-on-one or a close-range effort.

Under Puel, Bertrand has been given more attacking licence than ever. "It's a change - new tactics, new everything, to be honest," he said in October. "He likes his full-backs to attack and we're starting a lot higher up the pitch this season. He has a whole footballing philosophy, he likes possession and we have been controlling games more."

Indeed, Southampton's possession has risen from 49 per cent to 55 per cent this season. Bertrand is making fewer tackles and fewer interceptions, but he is more involved in their build-up play. According to Opta, almost 40 per cent of Southampton's attacks are directed down their left flank. It's the second highest ratio in the Premier League, and it's a deliberate tactic to unleash Bertrand.

With holding midfielder Oriol Romeu dropping into the backline and left winger Nathan Redmond cutting inside to use his stronger foot, the full-back has the freedom of the flank. He is enjoying more touches, making more passes and creating more chances than in either of his previous seasons at Southampton.

Bertrand's attacking influence has grown under Claude Puel

For Redmond, though, Bertrand's defensive influence is just as important. "He's always in my ear if I do something good or bad," said the 23-year-old recently. "To have someone strong behind you is a good feeling and you know defensively you have a little bit less to do. We're building a strong relationship and it's showing."

Redmond is not the only one to have praised Bertrand's character. Director of football Les Reed cited the defender's "great leadership qualities both on and off the pitch" when he agreed a new long-term contract in July, and his increased importance to Southampton's system suggests Puel feels the same way.

And so, two years on from cutting ties with Chelsea, Bertrand's progress continues. He returns to Stamford Bridge hoping to give his former employers a first-hand reminder of what they let go. If Southampton end up stalling Chelsea's title charge, Conte might find himself cursing his predecessor again.