"I should have been jailed for my tackles!"
The Spanish champions face city rivals Real Madrid in Saturday's Champions League Final in Portugal.
They will be watched by the Lisbon Lions for the first final to take place in the city since Celtic's famous 1967 triumph.
Atletico famously beat Celtic in a shameful 1974 semi-final clash, where three players from the Spanish club were sent off in a goalless draw in Glasgow, before they won the return leg to qualify for the final.
Argentine defender Diaz was shown red for a wild assault on the late Jimmy Johnstone.
Diaz, now 68, was banned for the final against Bayern Munich which Atletico lost and has finally come clean about that night in Glasgow.
He said: "I have cursed myself for years about the tackles I did, but I can't change anything.
"I meant no malice - I did it for the team to get the win, which we did, but it prevented me from playing in the final.
"Looking back over the years, I can now say those tackles I made were worthy of more than just a red card, I should have been locked up in jail for them!
"I knew I had gone too far, but we came for the draw at Celtic Park and got it."
And another Atletico star from that night in Glasgow who was sent off, Ruben Ayala, also admitted he'd gone too far.
He said: "We knew we had acted poorly.
"But we got three players sent off and still got a draw at a place like Celtic Park. We were a very good team."