The giants fell in the third round of qualifying, with Porto and Celtic headlining a day of casualties.
Celtic boss Neil Lennon refused to look for excuses after his side lost to CFR Cluj in Champions League qualifying.
Celtic again came from behind against CFR Cluj only for Abdel Billel Omrani to help to deny them a Champions League play-off spot.
Arsenal have bolstered their defensive options by signing left-back Kieran Tierney from Celtic.
David Luiz reportedly trained on his own at Chelsea as he looked to force his way out of the club before the transfer deadline.
James Forrest opened his goal account for the new season with a fine first-half strike in Celtic's 1-1 draw at CFR Cluj.
Celtic will meet Slavia Prague if they reach the Champions League play-offs, Ajax perhaps have an easier draw, and Porto face a big test.
Celtic issued a stern warning to their would-be title rivals on the opening day of the new season in Scotland.
Aleksandr Kulinits' own goal and Marian Shved's strike ensured Celtic will play Cluj in the Champions League's third qualifying round.
We round up the front and back pages across Europe as Monday sees the transfer rumour mill crank into gear once again.