Newcastle have signed goalkeeper Matz Sels from Belgian side Gent for an undisclosed fee.
Petrov, 36, is in remission after his battle with acute leukaemia and announced in April that he was planning to make a playing comeback.
Lafferty, 28, started twice for his country at Euro 2016, despite not starting for his parent club since January 2015.
Moussa Dembele has been confirmed as Brendan Rodgers' first signing as Celtic manager.
Nottingham Forest have appointed Philippe Montanier as their new head coach.
Borysiuk, 24, has signed a three-year deal with the Loftus Road club and joins from Legia Warsaw an undisclosed fee.