Marx was surprisingly released by Wrexham at the end of last season and played for the Saddlers in their friendly at Rushall Olympic earlier this month.
The youngster featured 10 times for Keates while the 40-year-old was in charge at the Racecourse Ground.
But he wasn’t in the Walsall team that took on Stoke on Saturday and won’t be part of the squad that takes on Villa tonight.
And while Keates said a move for Marx could be revisited in the future, at the moment the boss is concentrating on other targets.
“Olly has had one or two possibilities elsewhere but we can still revisit that,” Keates said.
“If I did do something, would I see him going straight into our team? No, not yet, he’s not quite ready for that.
“It would be a case of me trying to get a loan for him so he can go out straight away to help his development.
“It’s still something we can look at, it’s an option. But we are looking at one or two others at the moment.”
Walsall were dealt a hammer blow on Saturday when Kory Roberts suffered a double leg break in the friendly loss to Stoke.
Prior to that game, Keates had been on the lookout for a defender. And with Jon Guthrie now the only senior centre-back at the club, the boss admits he needs reinforcements sooner rather than later.
“We know it’s something we have to address,” Keates said.
“We were looking for a centre-back anyway and then we get the terrible news about Kory.
“We need to address it sooner rather than a later.
“We have got one or two things out there and I think things will open up a bit more now.
“There are players out there who have been going to the gym and training by doing their own thing – but now everyone else has reported back.
“They’ll be movement on that side and with the World Cup over, a lot of the under-23 players will now be available.
“We have got a couple we have looked at for loans.”