Black Cats boss' comments following Sunday's 2-1 home derby defeat to Middlesbrough sparked dismay from some fans.
Mannone, 28, sustained the problem in training and joins the Black Cats' growing injury list.
"The British manager is getting squeezed out," former Charlton and West Ham manager Alan Curbishley said in an interview with ESPN FC.
The Football Association has charged Burnley's Andre Gray with misconduct for homophobic tweets the striker posted four years ago.
Nabil Bentaleb's days at Tottenham Hotspur under Mauricio Pochettino look numbered despite being a fans' favourite at the club.
It appears that Henrikh Mkhitaryan has been deemed not quite ready for English football. Hopefully, writes Musa Okwonga, that will change soon.