The Gunners were thumped 5-1 in Prague on Tuesday.
Horseman later said: "A really disappointing day for me and for the staff, because I think we put a lot of effort into this game and then more for the players because I don't think they showcased what Arsenal's all about, particularly in the academy.
"There's lots we can all learn from it and we all get judged on and today is one where, rightly so, we should all be judged in a negative way because it just wasn't good enough."
Horseman, U18 coach Adam Birchall and U21 coach Max Porter addressed the players on the pitch after the final whistle.
"We told the players some home truths because we only ever tell them the home truths to help them," added Horseman.
"This is nowhere near the level. We'll look at ourselves because we're in it together and see what we can tweak. We've got two brilliant games, two tough games Bayern and Brugge and we'll try and put a few things right just so that this group can learn for future competitions."