The Leeds skipper knows from experience, having spent time on loan with Carlisle United, Hartlepool United and Chesterfield in the EFL while a youngster at then-Premier League Hull City.
Cresswell has come through the ranks at Thorp Arch and broke through into the senior side last season, getting a full 90 minutes at Elland Road against West Ham United on his Premier League debut. The England Under 21 international is highly thought of at Elland Road and considered a big part of their future plans, but he needs game time and Millwall can provide it.
Cooper hopes that Cresswell benefits from his time away as much as he did at a similar age, but he says Leeds will also gain from the move.
"For me it was unbelievable," the club captain told the YEP.
"Going on loan and playing games. Charlie is an unbelievable player and it will only benefit him, going away and playing men's football and being seen as one of the starters every week. It's a totally different mindset. How you prepare each game, how you stay fit. In the Championship, it's games every two or three days. It's about keeping yourself mentally in there and getting yourself up for every single game and it'll only benefit him and in the long run it will only benefit us as a club."
Cresswell, who has won league titles with Leeds Under 23s, has proved a popular character with the first team players at Thorp Arch. He was considered part of the senior squad by Jesse Marsch and made five appearances in the top flight last season. Marsch's predecessor entrusted the teenager with important roles late in two Premier League games to see out a draw and a win, while Marsch used him against Wolves in a thrilling 3-2 victory. Cresswell played a full part in the celebrations as his boyhood club survived the drop and then headed off on international duty, starting three of the Young Lions' four post-season games.
Millwall are winners in this deal too, according to Cooper.
"He's an unbelievable lad," he said.
"He's got a great attitude and I know he'll go there and deliver, that's the type of person he is. Playing men's football is all you ever want as a young lad and he's going to go do that. He's got everything to go and be a top player."