Matt Grimes joined Leeds United on loan this week.
The 19-year-old joined the Cherries on Saturday in a deal which could potentially pocket Leeds up to £10 million, according to The Guardian.
His exit was preceded by a flurry of incomings at Elland Road, with Grimes arriving at Leeds either side of deals to sign veteran goalkeeper Robert Green and young striker Kemar Roofe in a very busy 24-hour period for the Championship club.
By virtue of his transfer fee, it could be argued that Cook was one of United's best players, and former Blackburn Rovers man Grimes, joining on a season-long loan from Swansea City, admitted that it would have been nice to be the youngster's team-mate next term.
"Obviously I saw it today [the news of Cook's exit] and he shot off, " he said to LUFC TV. "I would have liked to play with him I think.
"Playing against him last year and from what I've heard he's obviously a very good player. I would've liked to see what we play like together but obviously that's not going to happen."
Losing Cook is certainly a blow, but it underlines the importance of now keeping left-back Charlie Taylor at the club.
The 22-year-old was arguably Leeds' other flagship player and losing both in one summer may be detrimental for their promotion aspirations next term.