was a surprise inclusion in the Lions starting line-up against Gravesend & Northfleet on Thursday night.
The Brazilian was given permission to play for Millwall in their opening pre-season friendly by the Gunners with a view to a possible loan move.
"Juan has come highly recommended from Liam Brady in particular", explained Lions boss Mark McGhee. "We've been having a look at him in training this week and are trying to see where he might fit in with us, as a left back, a wing back or as a winger.
"It's well documented that we're looking for a left-sided player and we'll see if Juan fits the bill."