"Before the game the boss gave us a good talk. We started well, in the first half it was even, after that I suppose their boss gave a similar talk, at halftime and they put pressure on. We knew that we'd suffer for a lot of the game," Kubo said after the match.
On loan from Real Madrid at Mallorca, Kubo said about scoring against rivals Atletico: "I'm here for Mallorca 100% but if I can help [Real Madrid] too, I'm delighted. But I'm with Mallorca and only thinking about Mallorca."
In an electrifying match in the Spanish capital, Atletico went in front through a Matheus Cunha strike in the 68th minute.
However, with 10 minutes left and Atletico looking in control of the match, Mallorca equalised with a header from Argentine defender Franco Russo, who outjumped Mario Hermoso from a free-kick and finished with precision past Jan Oblak.
Eight of the 15 goals conceded by Atletico this season have come when they have been defending set-pieces.
As the clock ticked down in stoppage time Kubo, on loan at Mallorca from Real Madrid, received a perfect through pass from Angel Rodriguez behind Atletico's defensive line and the 20-year-old Japan striker calmly slotted the ball under Oblak.
His team started strongly and created a few chances but Mallorca were not scared and put Oblak to work as he pulled off a great save to keep out a powerful shot from Lee Kang-in.
Atletico were more aggressive after the break and took the lead thanks to some fine individual play from Angel Correa who crossed for Cunha to finish inside the six-yard box.
Russo then levelled the scores before Kubo came on to hand Mallorca their first win at Atletico since 2006.
"The team wasn't good in the first half, in the second we were better, we found the goal, they got the equaliser from a set piece and then on the counter they got the second as we went for the win. We have to analyse itl a lot of things, right now in the heat of the moment there isn't much to say, when we look at it later we'll see," Atletico captain Koke said.
On Atletico's recent struggles, Simeone said: "It's hard to explain situations sometimes, there are times when things flow naturally, and in others they're more difficult, possibly we're in one of those moments. We have a squad I trust in absolutely. W e're in a complicated moment, it's better to have games coming up where we can turn this situation around."
Asked if his side were looking ahead to the Porto match, Simeone said: "No I don't think so at all, we couldn't control concrete situations, Oblak didn't have a save to make I don't think, I can't say much more, two shots on goal is what happened."