The Champions League quarter-finalists are understood to have fielded a weakened side in Marbella against the Hornets, who were on a five-day training camp.
Gianfranco Zola's men came away 2-1 winners as they continued their preparations for their Wembley showdown against the Eagles.
Deeney believes the run-out was a worthwhile exercise and will help ensure Watford are not off the pace on Monday.
"The boss does not like us having too long between games," said Deeney.
"It was only 45 minutes each and nothing major. There were no big tackles or anything like that. It was just to keep us ticking over.
"We worked hard over there and the fitness guys will make sure we are 100 per cent on the day. We don't want a week off and then play catch-up on the big day.
"Our aim is to be 100 per cent and be ready for Palace."