The outcome of the FA Cup final may be decided by which David Luiz turns up. Will it be the dominant centre-half of the semi-final, or the erratic defender we have become accustomed to watching for most of his career?
Luiz and Pierre-Emerick Aubameyang inspired Arsenal's win on their last Wembley visit, the Brazilian's performance especially eye-catching against Manchester City given the calibre of opponent and previous struggles against world-class attackers.
In a back five, Luiz was unrecognisable from the player we have seen struggle at the Emirates. It was comfortably his best game for Arsenal, probably one of the most flawless of his career. Few saw that coming after his display against City a month earlier.
I was very critical of Luiz during his first appearance of the Premier League restart, a heavy defeat at the Etihad Stadium.
He was directly responsible for two City goals and sent off. I called it as I, and the majority of supporters and reporters, saw it. I was provoked by its familiarity.
Luiz was a liability that night. It is not the first time we have said that. We could reel off a list of games when he was at the heart of a defence which capitulated, most memorably Brazil's 7-1 defeat by Germany in the World Cup semi-final in 2014.
It would be wrong to pin all the blame on one player for that, but since his breakthrough he has developed an unhealthy habit of being at the scene of defensive accidents, a member of a back four buckling under pressure.
The same player can point to his role in collecting an impressive list of honours. In the biggest game in club football - the Champions League final against Bayern Munich - Luiz was heroic in Chelsea's defence alongside Gary Cahill.
Six of the Bayern players beaten in 2012 were in the Germany team against Brazil in 2014. Those extremes sum up Luiz and how he has always been perceived.
After the Premier League game at City, I suggested we may have seen the last of him in an Arsenal shirt. Luiz's dismissal meant he received a three-match ban when his short-term Arsenal deal was about to expire.
There were eight Premier League games remaining of what, for him, had been a poor season.
I could not see how Arsenal would justify a contract. I was wrong, Luiz signing a one-year extension two weeks later.
As is often the case, when a pundit makes a criticism, those supportive or close to the player retaliate. No problem. The comments are not designed to hurt. Luiz's agent, Kia Joorabchian, led the response.
"He's a guy with a lot of experience and played under [Rafa] Benitez, [Maurizio] Sarri, [Antonio] Conte, [Jose] Mourinho, Unai [Emery] and now Mikel [Arteta]," said Joorabchian on talkSport. "If you speak to all those top coaches, especially coaches like Benitez, who had worked with Carragher and David, and ask him who he'd choose first, I think he'd pick David all day long."
I may have to double-check that one with Rafa but certainly Luiz should be better than I was. He is taller, quicker, technically gifted enough to play midfield and can strike a free-kick in a way I could only have dreamed of. What lets Luiz down is his brain freezes - loss of focus, poor decision-making and reading the game. So much of football depends on what is between the ears.
Joorabchian understandably came out fighting to ensure his client's contract was extended. The agent has the knack when it comes to negotiating, convincing Arsenal to take Luiz last summer, and to prolong his stay in north London into next season.
Then Manchester City manager Pep Guardiola joined in the rebuke of mine and Gary Neville's remarks.
"I laugh a lot at all the pundits that when they were a defender they didn't believe they made one mistake in their careers when they played 10 or 15 years" he said.
"I have incredible respect for his [Luiz's] career. He's an exceptional player with an incredible mentality and especially personality."
Lavish praise indeed, and fair comment on the outspoken ex-players. I am just surprised if Pep rates Luiz so much he has never signed him. Guardiola's pre-match comments were vindicated with Luiz's superb semi-final performance.
Luiz has worked with several top coaches. But how many have tried to offload him after a year or two? Including Mourinho? Why?
It is not because of Luiz's natural ability. It is because they did not trust Luiz in a back four. His best season was in a back five when he had the security to make an error and a team-mate can ensure it is not so catastrophic.
Arteta has mirrored what Conte did in his first year as Chelsea manager, identifying the players at his disposal and deciding a change of shape to three centre-halves fits Luiz's profile.
Luiz won the Premier League title under Conte in 2016-'17. He excelled that year in the middle of a back three.
Since then, old habits have returned. Luiz has conceded five penalties for Arsenal.
Just as against City, Luiz heads to Wembley needing to make amends. His career is too advanced to change minds about how reliable he is.
Those who think this is harsh, ask yourself this: which Luiz are you expecting at Wembley? Munich 2012, Brazil 2014, or FA Cup semi-final 2020?
Even Mikel Arteta cannot be 100 per cent sure.
© Daily Telegraph, London