Newcastle are closing in on the signings of Crystal Palace striker Dwight Gayle and Bournemouth winger Matt Ritchie, PA Sport reports.
Football fans have reacted angrily to a plan to allow Premier League clubs to field under-23 teams with up to four overage players in the EFL Trophy.
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.