United already have a deal in-place for Tosic worth around £8 million and intend to sign fellow midfielder Ljajic, 17, once he is capped by Serbia at senior level.
However, Ivan Golac, who played for Southampton and Partizan before managing Dundee United, told the Guardian:"I don't know of any Serbian player so young who has been a success in such a famous club.
"Many of our players rush to sign and then disappear. I think United have a plan to do the same as Chelsea did with young Rajkovic, and loan them out. Tosic and Ljajic will probably end up at other clubs as well. I'm afraid we lose Ljajic and Tosic as they move from club to club."