Mourinho had not lost a league home match since February 2002 when his FC Porto side lost 3-2 at home to Beira Mar, however, the loss to Sporting means Barcelona can move eight points clear at the Spanish first division summit with victory at Villarreal later on Saturday.
With just eight games left to play, Portuguese boss Mourinho admitted an eight point gap would effectively spell the end of their title hopes.
"Mathematically the league is not over but if the gap goes to eight points it is practically impossible (to win the title)," said Mourinho. "Luck is a part of football and our opponents had it all, while we didn't have any tonight.
"We had Cristiano (Ronaldo), Marcelo, (Karim) Benzema and Xabi (Alonso) missing and had to play a bit more direct tonight. We had lots of goalscoring chances from two or three metres but they just didn't go in.
"We know our limits and the players are dead tired. Today it was not a matter of defending more but getting the goal. If I wanted to draw 0-0 I could have done but that would have been a bad result."
Sporting coach Manolo Preciado, who had a public war of words with Mourinho earlier in the season, revealed his opposite number had been dignified in defeat, saying: "Mourinho congratulated all my players in the dressing room after the game."
Real had won their previous 14 league home matches prior to the game but the end of their 100 percent home record gives double champions Barcelona more impetus in the title race and comes ahead of a crucial Champions League quarter-final first leg match at home to Tottenham Hotspur on Tuesday.
"It was a defeat we didn't expect but now we have to get focused on the Champions League," said Portuguese defender Pepe. "We are still alive in the league and have to continue fighting to honour the shirt."
Mourinho has an injury crisis ahead of the Tottenham game and said it would be a gamble to play Portuguese winger Cristiano Ronaldo as he recovers from a hamstring injury.
"If (Cristiano) Ronaldo plays (against Tottenham) it is because he and I are taking a risk," said Mourinho. "The doctor said he is only ready for the Athletic Bilbao game (Madrid's next league match).
"Marcelo is out for two weeks and Benzema will only be back for Bilbao or the return leg (against Tottenham) in London. I will talk with the injured tomorrow (Sunday) and see if we will take a risk."