Sunderland announced they had relieved Grayson off his duties just 17 minutes after the club drew 3-3 with Championship basement-club Bolton.
Grayson took over the job in the summer after the Black Cats were relegated from the Premier League last season.
The summer transfer window also saw a huge overhaul of the squad that went down with eight players leaving.
But a run of just one win in their opening 10 Championship games has left them in the relegation zone with just 10 points.
However, Middlesbrough boss Monk, whose team faces Sunderland this weekend, feels Grayson has been unfortunate to lose his job.
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“I am gutted for Simon, he’s a fantastic manager and he’s a great guy as well,” Monk told talkSPORT.
“For me it’s mind-boggling. I think everyone knew how difficult that job would be this season – a rebuilding job and there was no better guy to do it than Simon.
“Especially at this level with the experience that he has.
“But in football strange decisions go on all the time so does it surprise me? No.
“But for myself, for Simon, I think it’s a poor decision.
“In terms of our game, what that means for us at the weekend, who knows but in derby games form usually goes out of the window.”