Barcelona dropped three crucial points in their bid for the title on Thursday.
A lowly Granada went home 2-1 winners against the Cules, completely damaging their chances of reducing the gap the gap to the league leaders.
“Our concentration in defence wasn’t as good in the second half,” Koeman acknowledged. “We created chances to score, especially at 1-0, but we lost our concentration.
“That’s why we lost, and they closed us down well. We have to accept that we lost, but we’re disappointed because this was a big opportunity.
“Now we have to be ready for Sunday. We’ve got five games left and everyone has a tough run-in. We have to win all five games to be champions.”
The setback won’t play on Koeman’s mind for long, though. The Dutchman was keen to move his focus onto getting back on track.
“We’ve won a lot of games,” Koeman added. “We’ve fought a lot. Despite today’s defeat, we have to keep going.”
Barcelona could have snatched the lead away from Atletico Madrid while both teams dropped points this week. But now, they sit two points away, level on points with Real Madrid but behind due to an inferior head-to-head record.