Josh Ogunnaike scored for the Gunners after Olympiakos had gone 2-0 ahead. The visitors' goals came after Louie Copley was shown a red card.
Horseman told the club website, “I said to the players straight after the match that you can’t give away goals like that at this level. Both goals we conceded were really poor, just individual mistakes, and they came at a time when we were starting to control the game. Once the red card happened, it shook us, and suddenly we were 2-0 down. At this level, that’s always going to be tough.
“But in the second half, I felt immense pride. It’s probably the best I’ve seen some of the players since I arrived. The wide players caused Olympiacos endless problems, and Kyran Thompson, who is still only an under-16, linked play brilliantly in the pockets.
"In the end, though, we’re disappointed. We should have at least drawn, maybe even won if we’d been more ruthless in the final third.”