The Giallorossi were held to a goalless draw at home to Cagliari on Monday, a result that prevented them from reclaiming the top spot in Serie A.
The outcome was Roma's third-straight draw and left Garcia's side in second place, one point adrift of league leaders Juventus.
"We did well against Cagliari," Garcia said to Gazzetta dello Sport. "The important thing is that when we don't win, we don't concede goals and hence, we don't lose.
"We played a good match and we didn't get three points because (Cagliari goalkeeper Vlada) Avramov had a fantastic game.
"Our main problem has been that some of our players are not at the same physical level as others.
"(Daniele) De Rossi was only able to train with us for one session as he was involved with the Italian national team last week and the same goes for other players.
"When there is an international break , there tends to be a drop in form when returning to the competition and that affects the performance of the team."
Roma travel to Bergamo to face Atalanta on Sunday.