Claude Puel called for Southampton to improve their concentration levels after his side were beaten 1-0 at Sparta Prague in the Europa League on Thursday.
An early volley by Sparta's left-back Costa Nhamoinesu proved to be the winner as Southampton struggled for penetration despite dominating possession.
Southampton will have to either beat Hapoel Be'er Sheva or draw 0-0 in their final game at St Mary's in a fortnight to go through and Puel said his team have to maintain their focus.
"It's a bad result," Puel said. "We made a good start in the game, but then from the first free-kick we had a goal against us and this is hard to accept.
"I think we played a good first half without a clinical edge, but after we lost the structure in the second half to keep our calm.
"It was a good position for the other team, because they made it difficult for us to find solutions on a bad pitch, but that is not an excuse. It was not enough to win this game."
Southampton are winless in three games, scoring just once in that run, and Puel called for a renewed effort from his side ahead of Sunday's Premier League game at home to Everton.
"Now it's important to keep our concentration," Puel said. "We know at the beginning of the night a draw was not sufficient.
"Now it's a final game in front of our fans. We have to play a very good game for the next game at home to make sure we qualify, for the players and fans.
"Of course tonight it is disappointing. We now have to keep a good spirit for Everton on Sunday."
James Ward-Prowse said Southampton must now focus on getting the positive result they need in their decisive final clash against Hapoel.
"We are disappointed not to have won the game, but now we have to perform against Hapoel at St Mary's to ensure we go through," he told the club's website.
"We have to try and make St Mary's a fortress for when they visit. We've done well in Europe at home and we need to try and make sure we put this result right."