After allowing Branislav Ivanović to head in a late corner in Saturday’s 2-1 home defeat to Chelsea, they lost 2-0 at AFC Bournemouth to a pair of efforts that came about from Matt Ritchie free-kicks.
Having had a reputation all season for being so strong at defending set plays, Koeman says Saints must now rediscover their form.
"It’s disappointing. Normally we defend set pieces well, and it's now three in a row that have come out of free-kicks and corner kicks and we need to be sharper out of those situations,” Koeman told Southampton’s official YouTube channel
"It's always difficult to explain. Maybe it's also the quality of the free-kick of the opponent, but you need more responsibilities against your opponent and we lost a little bit of sharpness in that.
"We need to react. At least we need to be more aggressive and if you are more aggressive on the pitch you play better football.”