Goals from debutant Zat Knight and Gabriel Agbonlahor earned Villa the win which denied Chelsea the chance to return to top spot.
It was their first league defeat since losing to Liverpool at Anfield in January.
"The other teams have spent lots of money and brought players in to bolster their teams. Villa are one example but you can find a lot of other examples.
"It will be more difficult to play against these teams because they are better. The league is open but it is the first weekend of September and we are only five matches into the season."
Mourinho said: "Aston Villa played well, Chelsea played well and in my opinion a point each would have been a fair result.
"But Chelsea made a mistake you cannot make. We are very organised in defence at set pieces.
"But a game that we were in control of after a solid first half changed with that goal early in the second half. After that, Villa were well organised, showed great spirit and had the crowd behind them.
"The second goal means nothing to me in the context of the game. That can happen sometimes in the last couple of minutes when you are pushing forward.
"I accept the result but don't think it's fair - but Villa did not make the sort of mistake that we made."