Romanians Astra handed West Ham United their first competitive defeat at the London Stadium as Slaven Bilic's team were beaten in the Europa League playoffs on Thursday.
"The disappointment is massive," Bilic told reporters after Astra eliminated West Ham in the qualifying rounds for the second year running following a 1-0 away win and a 2-1 aggregate victory.
"It is a huge frustration because I said before the game that this was one of the most important games of the year for us. There was not enough movement, we played very slowly and we didn't take the chances we should have taken."
Filipe Teixeira scored the only goal of the night on the stroke of halftime with Astra's first on-target shot of the match.
West Ham are in their first season at the 60,000-seater London Stadium, formerly the Olympic Stadium, after spending 112 years at the Boleyn Ground.
Fenerbahce, who appointed much-travelled Dutchman Dick Advocaat as coach earlier this month, beat Grasshoppers 2-0 in Zurich as the Turkish club secured their spot in the group stage with a 5-0 aggregate triumph.
Newcomers Sassuolo followed up their 3-0 first-leg victory with a 1-1 draw in Serbia as they eliminated 1991 European Cup winners Red Star Belgrade.
The Italian side, who qualified at the expense of AC Milan, were promoted to Serie A for the first time three years ago and are making their European debut this season.
Sparta Prague substitute Jakub Brabec netted three minutes into stoppage time to give the Czechs a 3-2 win on the night, and on aggregate, against Danes SonderjyskE Fodbold.