Sir Alex Ferguson has warned Rio Ferdinand and Nemanja Vidic they no longer have guaranteed places in the Manchester United side.
Vidic could return from injury to play alongside Ferdinand at Liverpool.
The duo have not played together at the heart of United's defence since the 2-1 win at West Brom on the opening weekend of the season.
Captain Vidic has been out since August with a calf strain, while Ferdinand has only made three Premier League starts.
In their absence Phil Jones, Chris Smalling and Jonny Evans have impressed during their unbeaten start to the campaign.
Ferdinand, 32, has struggled with injury during the last two seasons and has been linked with a move to the MLS. He was also left out of Fabio Capello's England squad for the 2-2 draw with Montenegro.
Ferguson said: "Rio and Nemanja are well aware they are going to have to play well to keep their places.
"Competition is so fierce now with these young lads coming through. Rio is back to his best form. He has trained really well and his experience will be vital.
"The England thing is unfortunate because he missed a couple of appearances through injury and maybe Fabio is looking to the future?
"Some managers think that way. There is nothing wrong that."
Vidic, 29, who returned to action in Serbia's 1-0 defeat to Slovenia in midweek, has revealed he is considering retiring from international football.
Ferguson added: "I've spoken to Nemanja and it was probably the right type of game to come back in.
"He had to get a game sometime. He was training very well before that game and has come back feeling fine."