GALLERY: Springboks v England, second Test match-ups
England beat the Southern Barbarians 54-26 in Kimberley on Wednesday, but were made to work for their points in a match that was never dominated by the international side.
The English struggled at the breakdowns, with flank James Haskell receiving a yellow card for repeated infringements by Kaplan.
"We left ourselves short and, as a consequence, spaces opened up and they [Barbarians] scored four tries," Lancaster said.
"We pride ourselves on our defence and we're not happy about that. It is something we will have to rectify.
"We are delighted to get the win though, scoring eight tries in the process, but credit to the Barbarians side."
Lancaster felt his team took several positives from the midweek clash, but cited discipline as an aspect that needed attention.
"They absolutely made us work for everything and we learned one or two lessons along the way ourselves," he said.
"There are definitely some areas to work on going into Saturday and also next Tuesday."
Lancaster made the trip to Kimberley to see whether some of the players in his greater squad could still be utilised at Test level, gaining insight into his pool of players.
"We are nearly there in terms of team selection. There are one or two slots we are still considering, and will consider overnight, but I think we are pretty much there now."
Veteran scrum half Danny Care was lauded by Lancaster after his solid performance.
"I think he (Care) did really well. He set up wing Christian Wade for a good try in the first half and has a good intuitive sense of where there is space on the field," Lancaster said.
"I'm very pleased with his performance. He lasted the full 80 minutes and I'm pleased that he's got this game under his belt. It's been a while since he last played."
The England coach was also impressed with centres Jordan Turner-Hall and Anthony Allen going forward on the attack.
"Both Jordan and Anthony showed their strengths. Anthony particularly has got great hands in the middle, while Jordan worked hard at specially the breakdowns. He was good technically and cleared the ball pretty good."
England struggled in the last quarter against the Springboks, but against the Baabaas, they lasted the entire 80 minutes.
"Playing a good last quarter is something we discussed beforehand and I think we realised that goal against the Baabaas and the boys watching from the stand would have learned a lot from that effort."