George Saville has joined Middlesbrough on loan ahead of a big-money move in January.
The Northern Ireland midfielder has moved to the Championship's second-placed club initially on loan, the move completed ahead of today's 5pm deadline.
With the transfer window for permanent deals closed earlier this month, Saville can only complete a short-term move but that will be made permanent in the new year.
The deal is thought to include a locked-in £8m fee when the January window opens. That would be the highest fee Millwall have ever received for a player.
Saville was one of the stand-out performers in the Championship last term as Millwall just missed out on promotion. He had been linked with Leeds and Aston Villa earlier this summer.
He is seen by many Northern Ireland supporters as the midfielder successor to skipper Steven Davis. The 25-year-old has five caps, having made his debut in October.