Inter Milan have signed Brazilian defender Joao Miranda from Atletico Madrid.
Ipswich have signed winger Ainsley Maitland-Niles on a season-long loan deal from Arsenal.
The Scotland international has been a shining light in a disastrous season for the Bluebirds, whose relegation to the Sky Bet Championship was confirmed on Saturday following their 3-0 defeat at Newcastle and Sunderland's win over Manchester United at Old Trafford.
Cardiff have banked 30 points from 37 games under previous boss Malky Mackay and current manager Ole Gunnar Solskjaer, who confirmed on Saturday that he is planning to stay at the Cardiff City Stadium for next season.
Captain Steven Caulker and record signing Gary Medel are believed to have clauses in their contract which allow them to leave this summer if a certain valuation is met.
But City's player of the season Marshall has reportedly caught the eye of Tottenham and Liverpool following a string of superb displays - and is rated in the £6million bracket having been signed by Dave Jones for just £500,000 from Norwich in May 2009.
The 29-year-old seems keen to win promotion back to the Premier League, though, despite admitting the players are "devastated" to have lost their fight for survival with one game remaining, albeit against Chelsea at home on Sunday.
"It is very disappointing, the boys are absolutely gutted to be relegated," Marshall told Wales Online.
"It's not been enough in the end to keep us up. We expected more from the season and we've obviously been left very disappointed by this.
"There was not a lot we could say in the dressing room afterwards. Everybody has just been left devastated.
"It's going to be a hard couple of weeks now and we're just trying to get out heads around it all. The manager has asked us to do that. We've got a squad in there that won the Championship quite comfortably last season.
"We've got to get our heads right and start again. We had fighting spirit in abundance last season.
"We added players to it which is difficult, but we managed to keep that."
Marshall, who was named number one in Monday Night Football's Team of the Season, added: "There's been a lot going on around the club, but the players have stayed focused.
"But we've come up short. We've not been good enough in games and there’s no complaints about that.
"We just need to stick together and go again next season. We definitely believe we can bounce back.
"We've got better players now than we had last season, but the attitude will need to be right.
“We will need to start the season well. We'll be disappointed, but we need to pick ourselves up."