Garry Monk has described the decision to sack Simon Grayson as "mind-boggling".
Sunderland parted company with Grayson just over a quarter of an hour after the Black Cats scraped to a 3-3 draw with bottom-of-the-table Bolton Wanderers at the Stadium of Light.
There were calls for Grayson to be sacked from sections of the home support during the game and the club's hierarchy wasted little time post-match.
Grayson took charge in July and was manager of Sunderland for only 125 days but he managed only one league win since taking charge and the club is third bottom in the Championship.
Grayson was axed ahead of the Tees-Wear derby against Middlesbrough at the Riverside on Sunday and Boro boss Monk has reacted strongly to the sacking.
Monk said he was "gutted" for the former Blackpool, Leeds United and Huddersfield boss and said the Sunderland job was always going to be "extremely difficult" following relegation from the Premier League.
"I’m gutted for Simon," said Monk, in the aftermath of Boro's 3-1 win over Hull City.
"Simon’s a great manager, and a really good guy as well.
"Especially at this level, I don’t really understand the decision. Everyone knew what he was going to be walking into.
"It was always going to be extremely difficult, and it was always going to be a transition season for Sunderland.
"There was no better man to take that job. Does football surprise you? Not really.
"But in terms of what’s happened, it’s a bit mind-boggling for me.
"Simon was the perfect candidate and I feel for him. It’s not a nice thing, and it’s undeserved if we’re all honest about it."
Sunderland travel to Boro on Sunday lunchtime.