From the start of the season we've been thinking that the key to Chelsea's title race was keeping Diego Costa fit. He's the man who has delivered goals by the bucketload, after all.
Then when Jose Mourinho's side went and lost at Newcastle, there was a different view. It was the first game when Nemanja Matic went missing - so maybe it was the Serbian midfield enforcer who was the key player.
Now we know different. It is John Terry, at 34-years-old, who remains the heartbeat of the side at Stamford Bridge and without him there are no guarantees about what the rest of the star-studded line-up can do.
That above all else was the lesson from Saturday's incredible FA Cup collapse against Bradford. For the poor souls who thought they would pick up an easy few bob by matching £44,000 on Chelsea to win at 1.011/100 after they had gone 2-0 up, it's the thing to check next time. Is the Captain, Leader, Legend on the pitch?
Now I know that Mourinho, in his after match analysis, tried to insist that Terry's absence wasn't the key factor. "We played Bolton, Shrewsbury and Watford without John and the team coped well," he argued.
But in none of those games were Chelsea facing the sort of spirited comeback that Phil Parkinson's Bradford began to produce from nowhere at Stamford Bridge on Saturday. That was when the leadership of Terry was needed, and that was when there was nobody else to provide it.
Of course there were powerful personalities on the pitch. I spent last night at The Savoy Hotel where the Football Writers' Association honoured Didier Drogba. Petr Cech gave the tribute speech and they both talked about the winning ethic they had learned under Mourinho's management.
But on the pitch a goalkeeper and a centre forward - neither of whom will be playing in the majority of the games in the campaign to justify the current odds of 1.271/4 for the Premier League title - can only do so much. It is at the heart of the defence where Terry's powerful voice makes the biggest difference.
I suspect Mourinho's public utterances claiming Terry's absence wasn't a crucial factor are at odds with his private views. He knows he can't afford to send a message to the dressing room that Terry is so vital. He's played 29 of Chelsea's 34 games so far, but at 34-years-old there is always the risk that the weight of matches will cause him to break down with an injury at some stage.
If that happens any words suggesting his team cannot manage without their captain would come back to haunt Mourinho and he's too clever to take that risk.
Regardless of what the manager says in public, Terry's will still be first name on the team sheet for Saturday's title defining clash with Manchester City - who have their own problems hoping they can rush Yaya Toure back from the African Cup of Nations if the Ivory Coast get knocked out.
And you can be sure he will be back in the side too for tomorrow night's Capital One Cup semi-final second leg against Liverpool, making Chelsea rightly the 1.654/6 favourites to win in 90 minutes and 1.364/11 to qualify.
Mourinho might pretend otherwise, but the FA Cup turmoil this weekend says it loud and clear that, whatever the stellar talents around him, John Terry remains the key to a Chelsea title success.
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