Dons’ 46-0 defeat saw them drop a place to fifth in the standings.
“I didn’t see that coming,” said Horne.
“We’d had a really good week’s training and we spoke about doing all the right things but one thing you can’t control is what happens when the players turn up on the day.
“But credit to Crusaders; they did a really good job on us. They didn’t make many errors in the first half and built pressure and executed well when they got into good areas.
“Basically, they did everything that we had spoken about doing to them to us.
“They are probably the biggest team in the competition and we didn’t handle them well in the first and as a result we came in 28-0 down.
“I thought we did early in the second half and we gave ourselves at least a chance to post some points but we lacked organisation and a cutting edge and didn’t create a great deal.
“Although they finished worthy winners I think it would have been a lot closer had we played in the first half like we did in the second."
He added: “Obviously a defeat like that hurts because we wanted to back up the performance against Oldham last time out and we thought we could do that and we didn’t. It’s certainly a case of two steps back after one step forward.
“We’ll sit down as a coaching staff and work out what when wrong and try and fix things in training, but we don’t have much time to do that.
“It looks as though we need to strengthen the squad at this moment in time and we also need some bodies back.
“We are certainly missing Benny Johnston, who has two more games of his five-match ban to complete, because we are lacking his run threat with the ball.”
The Dons have had an inconsistent start to the season, losing heavily to Crusaders and Swinton Lions, losing at Keighley, beating Oldham and running out comfortable winners against West Wales, Cornwall and Midlands Hurricanes.
They travel to London Skolars this weekend.
Skolars, like West Wales and Cornwall, have lost their opening six games of the season.
“They aren’t the side they were last season when they proved a surprise packet," said Horne.
“It doesn’t surprise me where they are in the league after losing so many players to London Broncos.
“But it’s never an easy place to go down there with it being a long journey and a narrow pitch which makes it more of a forward battle, so we certainly aren’t taking things for granted.”