" at the start of the Formula 1 season.
He said the start to the season had been "tough", the team were "relatively concerned" and their difficulties had "caught us by surprise".
Ferrari lagged behind Red Bull at the season-opening race in Bahrain.
Sainz's team-mate Charles Leclerc starts this weekend's Saudi Arabian Grand Prix with a 10-place grid penalty because of reliability problems.
"We have faced an issue with race pace and reliability that we did not expect," Sainz said. "It caught us a bit off guard.
He said the start to the season had been "tough", the team were "relatively concerned" and their difficulties had "caught us by surprise".
Ferrari lagged behind Red Bull at the season-opening race in Bahrain.
Sainz's team-mate Charles Leclerc starts this weekend's Saudi Arabian Grand Prix with a 10-place grid penalty because of reliability problems.
"We have faced an issue with race pace and reliability that we did not expect," Sainz said. "It caught us a bit off guard.