Martin O'Neill feels he should have been given more time at Sunderland after helping them avoid relegation last season.
The 61-year-old's tenure at the Stadium of Light lasted 15 months before a 1-0 home defeat to league leaders Manchester United last Saturday left the club just a point off the relegation zone.
O'Neill was dismissed later that evening, with Paolo Di Canio appointed as his successor 24 hours later, and the former boss said he did not deserve to be dismissed.
"I am still pretty disappointed, frustrated as much as anything else but life goes on," O'Neill told the BBC.
"I'm in the business now where I think very little shocks you about professional football especially in the last 10 years. You can lose a job in management if your tie doesn't fit your suit.
"With the experience I have had over the years ... also coming into the football club at a time when the club was pretty well on its knees. I believed I saved it from relegation last year and I thought that opportunity should still have been afforded to me."
Sunderland are in 16th place with eight games remaining and face difficult fixtures against the likes of Chelsea and Tottenham, as well as relegation rivals Aston Villa, Southampton and bitter rivals Newcastle.