The London club suffered their ninth defeat of the campaign when they lost to Leicester City 2-1 at King Power Stadium on Monday.
The reigning champions are currently placed in 16th position on the league table but Matic is struggling to find a reason for their woes.
"I cannot explain why consistency has been so difficult to achieve," Matic told the club's official website.
"I don't know what happened to us [against Leicester]. In the game against Porto, a very good team, we played very well, we scored two goals and created chances, but we were like another team on the pitch against Leicester.
"We have to say congratulations to them, they fought for every ball, and they were very good. They had confidence and it is easier to play when you are first in the table and you are at home. We didn't start badly, we controlled the ball, but after the second goal it was very difficult for us to recover and we didn't use the chances we had.
"It is one more game that was not good for us. We are sad because of the defeat but we have to keep going, to try to win some games and to recover from this situation which is not easy for us. We have to try to be better than we were."