NEIL Harris praised George Saville after he made his debut for the Lions in the 2-0 JPT win over League Two Northampton on Tuesday night.
Millwall signed the midfielder on a 28-day emergency loan from Wolves this week and Harris said the deal could be extended further.
Saville, 22, played three games for the Lions on loan from Chelsea at the end of 2012-13 and the following season started 33 times for Brentford as they won promotion from League One.
He also helped Bristol City to promotion last season after Steve Cotterill brought him to the club on loan last January.
Harris said he came "highly recommended" by former Millwall boss Kenny Jackett, who signed him for current club Wolves in August 2014.
Saville produced a physical, energetic display against Chris Wilder's side as Millwall progressed to the area quarter-finals in a competition in which they last reached the final in 1999 when they lost 1-0 to Wigan at Wembley.
Millwall only confirmed the signing of Saville hours before they played the Cobblers, but with five midfielders missing through injury, suspension and international duty he was thrust straight into competitive action by Harris.
The Millwall boss has used his connection with Jackett to bolster his side's midfield in the short term, but could seek to negotiate a longer deal after the initial period ends.
Harris said: "I thought he was excellent for a boy who's not played a lot of football and come straight in having not trained for three days.
"He gave us a little bit of aggression in the middle of the park but also composure with the football. He comes with good recommendations from some good football people.
"I was really pleased with him. That's the standard he's set now and he's got to keep to that standard and improve as his match fitness improves.
"We'll see how the 28-day period goes. It depends on Wolves and depends on how we're going in the meantime."