Nemanja Vidic and Yuto Nagatomo were on the scoresheet for Inter in the International Champions Cup clash at Lincoln Financial Field in Philadelphia.
While he said the tour of the United States was about fitness, Garcia was displeased by his team's showing against Inter.
"Everyone could've done better today and we should've moved the ball around much faster. We had too many touches, making our play predictable and easy to read," he said.
"We played against a well-organised Inter side and never managed to catch them out of position. This must act as a real warning for the team. It's a friendly, fine, but when we are in a competitive match we cannot play like this.
"There was almost nothing positive. It's a very long road before we can be ready for the start of the season."
In further bad news for Roma, Leandro Castan suffered a hamstring injury early on.
Garcia was unsure how severe the injury was, but the Brazilian centre-back could miss the start of the Serie A season against Fiorentina on August 30.
"The really bad news is Castan's injury. It is a problem and we hope it isn't too serious," he said.
"We have a month before the start of the season, but a muscular injury can be a lengthy one."
