Millwall boss Neil Harris feels Harry Toffolo's arrival will give the club a "different option" at left-back.
The 22-year-old joined the club after putting pen-to-paper at Calmont Road on Monday morning, with the former Norwich City man stating he "can't wait to get started."
Harris, meanwhile, told millwallfc.co.uk that the deal gives both parties a chance to weigh up their options.
"I'm really pleased. Harry's a player that we've followed from afar for a couple of years now," he said.
"He's an attack-minded full-back and he likes to get forward. His age in the modern game dictates how he plays.
"What the deal does is give us an opportunity to look at Harry and vice versa until the end of the season, with a view to something a little bit longer.
"We've got good age and experience in that position, but maybe there's a situation where we need a young and fresh one coming in to give us a different option, similar to what I've got at right-back with Conor McLaughlin and Mahlon Romeo."