The champions make the short trip across Madrid on Monday to open their league campaign, with Simeone having insisted in the build-up that they face a huge task to repeat the feats of 2013-14.
Atletico won La Liga on the final day of last term, finishing three points ahead of Barcelona and Real and in the process breaking the duopoly that has dominated the Spanish top flight since Valencia's title success in 2004.
They go into their season opener brimming with confidence following their 2-1 aggregate win over bitter rivals Real Madrid in the Spanish Supercopa, but the Argentinian is playing down expectations.
He said: "Barcelona, even in a bad season for them, almost won the league. And Madrid as well.
"If they had beaten Valencia [in their 35th game of the season] they would have been top.
"It is very difficult to compete across so many fixtures. But we are competitive over 90 minutes."
Atletico's recent history against Monday's hosts suggests they should at least be expected to start with a win and, although they lost 2-1 at Vallecas in February 2013, that is their only defeat to Rayo since 1999.
Simeone's men have also netted nine times in their last two meetings with their Madrid neighbours, but Rayo's close-season transfer dealings at least give them a reason for optimism.
Coach Paco Jemez has brought in Gael Kakuta, Javier Aquino, Jonathan Pereira, Leo Baptistao, Lica and Abdoulaye Ba have all on loan, while the free transfers of Jozabed and Alejandro Pozuelo add depth to their creative midfield.
Jemez will also take encouragement from the fact that he only has one certain absentee for the clash, as defender Antonio Amaya - a new arrival from relegated Real Betis - is expected to be out until mid-October with a tear in his thigh.
It remains to be seen, though, whether Baptistao will play - with both Atletico and Rayo staying silent on any potential clauses over facing his permanent employers in his loan deal from the champions.
Simeone has a few selection issues to contend with as Raul Garcia is suspended and Arda Turan (hamstring) could miss out, but the majority of his squad is available.
The former Argentina international may not be present on the bench, however, after he was sent off during the Supercopa win - with reports suggesting that he could be set for a ban.
