Benfica coach Jorge Jesus insists he cannot perform "miracles" after suffering a miserable pre-season campaign.
The Portuguese giants have lost all four of their pre-season matches - to Sporting and Ajax in July, and Arsenal and Valencia this weekend.
The defeats followed the departure of a number of first-team players including winger Lazar Markovic to Liverpool, goalkeeper Jan Oblak to Atletico Madrid, defender Ezequiel Garay to Zenit St Petersburg and midfielder Andre Gomes to Valencia.
"There is no team in the world who lose eight players and in four weeks can do miracles with his team," Jesus told reporters. "I am Jesus... But Benfica need time.
"Besides the players who stayed with us, the new players have found it difficult to adapt.
"They have not yet understood the requirements of the team, but they have been working under an extreme physical training regime. This has caused players to not perform to their potential."
The Aguias were beaten 3-1 by Valencia on Sunday a day after being thrashed 5-1 by Arsenal. They took an early lead through Derley but a second-half comeback for last season's Europa League semi-finalists left Jesus' men on the losing team once again.
"Of course, even in pre-season it is better to win than lose, but in these games we can take risks and experiment," Jesus added.
"So I think these four game were important and we will start making conclusions on Monday in order to improve in games ahead."
Benfica face their first competitive game of the season against Rio Ave in Sunday's Supertaca ahead of the new league season.