After two wins over Puerto Rico in London, one of which was behind closed doors, Britain lost 88-66to Finland in Helsinki before travelling to Crete to face Greece.
They couldn't get back to winning ways however, falling 95-62, with more warm-up matches scheduled against differing opposition in Poland and Sweden.
Britain begin their EuroBasket campaign against Israel on September 4 in Slovenia and Achara insists they need to resolve their on-court issues swiftly.
"We have to learn quick," said Achara, who stood in as captain for Drew Sullivan against Greece and recorded a team-high 15 points. "We're going to get thrown into tough situations and as professionals; we have to respond to that situation. We have guys who are learning but they'll need to do it quickly."