Why John Terry-Gary Cahill Is Chelsea's Best Centre-Back Partnership

John Terry and Gary Cahill have plenty in common.

They've won every major honour in club football, domestically and in Europe. On the international stage, both have also captained England at Wembley—Cahill reaching that milestone this weekend against Estonia.

Terry and Cahill are also Chelsea's best defensive partnership. For now.

Where things have gone awry for Chelsea this season is anyone's guess. There have been so many major issues from front to back that it's difficult to pinpoint one area where things have gone wrong collectively.

If we look at the defence in isolation, however, it's clear the form of Branislav Ivanovic has been a major factor.

The Serbian has disappeared this season, with the player we've seen at right-back being only Ivanovic by name.

Since May, Ivanovic seems to have lost all his ability as a footballer. He's suddenly weak in the air, opposition wingers are skipping past him like he's a training-ground dummy, and even in attack he's offering little.

In the recent Champions League loss against Porto, Ivanovic couldn't pick out a team-mate with even one of his many crosses in the game.

It's been torture for Jose Mourinho as he watches on from the dug-out, seemingly powerless to stop the slump his team finds itself in.

Whatever Mourinho has tried has led to failure.

Every combination at the back has seemingly created a different problem, but with Ivanovic playing every minute in the Premier League thus far, many will point to that as Chelsea's biggest.

That would be a sensible diagnosis given Chelsea have conceded an average of two goals every game in the league this term.

By comparison, they conceded 32 in 2014/15, so despite being just a quarter of the way into the new campaign, they're already halfway to that goals-conceded tally.

It doesn't make for comfortable reading.

Ivanovic aside, Mourinho's insistence on rotating his central defenders hasn't helped matters.

At different stages this season we've already seen three different combinations. It started with Terry and Cahill; halfway through the 3-0 loss to Manchester City, it was switched to Cahill and Zouma; for the next outing against West Bromwich Albion, Mourinho fielded Terry and Zouma.

When Terry was sent off in that match, Cahill was brought on to partner the young Frenchman.

Like Chelsea's first eight games, the defence has been all over the place in more ways than one. Mourinho can't decide which setup is best, and the players have been erratic as a result.

The situation isn't helped when key players like Ivanovic and, further forward, Nemanja Matic are disastrously off form, either.

Mourinho needs to return to the formula that gave him so much success last season.

It was the Terry-Cahill partnership at the back that gave Chelsea their foundation, and the manager needs to show some more faith in it.

There are other changes needed besides that, but getting that pair back together on a consistent basis would be a good place to start.

Of course, the 3-1 drubbing at the hands of Southampton as we headed into the international break doesn't breed confidence in that notion.

Saints were awesome that day, cutting Chelsea apart at will to record what should have been a more emphatic victory.

But look at the facts: It was mistakes elsewhere that led to Southampton's goals, with Sadio Mane's strike coming as a result of Terry being left isolated and out of position.

Mourinho has commented already this season that he's preferred Zouma to Terry at times because the Frenchman's pace has allowed for Chelsea to play a higher line and attack teams to win matches.

Only once has that worked, though, when Chelsea beat Arsenal in a game remembered more for controversy than Chelsea's performance.

Chelsea have been too open defensively, which has allowed runners to find space and punish them.

Now it's about closing those gaping holes and making the rearguard as watertight as it can be.

He may well be approaching his 35th birthday, but there isn't anyone better at implementing that policy than John Terry.

The Chelsea captain's positioning remains impeccable—despite the odd blip—and with the authority he boasts, he's the man to get Chelsea's defence out of this crisis.

Playing a more experienced head alongside him in Cahill, it's the sort of combination Mourinho needs.

In times such as these, the solution many call for is a common one. It's the usual line of teams going back to basics to get their house in order, which is what Chelsea are desperate for.

What those basics are varies from club to club, with the point being they need to get back to what they do best.

For Barcelona, that may be maintaining possession and sucking the life from opponents by simply starving them of the ball. For Chelsea, it's battening down the hatches and welcoming teams on to them, soaking up the pressure to counter at pace.

The only way they can start doing that is by rediscovering their defensive prowess. It's what delivered them success last season, and it's what will get them out of the hole they find themselves in.

For all his talents, Zouma isn't quite capable of being the man to deliver that. Yet.

That's not to criticise the youngster, but it's more a reflection of where he is in his career. He comes with a profile that is only heading upwards, although he remains raw.

Watching his last-ditch tackles and excellent recoveries looks good on the surface—indeed, it can be exciting at times, getting the crowd on their feet—but it serves to highlight where his game must improve.

Zouma needs to develop his positioning and understand the game better. There are times he can be too eager, and that's where problems can develop, leaving him to play catch-up and make those lunged interceptions.

Cahill and Terry are wise enough to pick and choose their battles, passing them onto others around them.

A good defence is about good shape and discipline, which Zouma doesn't offer all the time.

And besides, if Mourinho's preference for Zouma over Terry has been to play that higher line and chase games from the off, it goes against every fundamental he has preached throughout his career.

It's hardly going back to the basics.

This Chelsea team isn't set up for that; where they find themselves now doesn't call for it, either.

Before Chelsea can consider themselves to be dominant against opponents, they need to accept where they are now, which is 16th in the table and closer to the relegation zone than they are the Premier League's top four.

Basics, strengths, fundamentals—call it what you will—Chelsea must rediscover theirs before this season can come close to being salvaged.

Start with Terry and Cahill and go from there, Jose.

Garry Hayes is Bleacher Report's lead Chelsea correspondent. All quotes were obtained firsthand unless otherwise noted. Follow him on Twitter @garryhayes