Allardyce was surprisingly dismissed as Rovers boss on Monday with his team 13th in the Barclays Premier League, with club owners Venky's stating he did not fit in with their "long-term vision".
But Coyle, whose side beat Blackburn 2-1 on Sunday in what turned out to be Allardyce's final match in charge, was unimpressed.
He said: "Sam Allardyce has done a fantastic job at Blackburn Rovers - as he has done everywhere else - because he is an outstanding manager, and shocked is probably an understatement.
"The way it was done, we know in football that that happens, but it doesn't make it any easier. They are going to have to go some, I would suggest, to get a better replacement than what they had in Sam Allardyce.
"I think Blackburn have lost an outstanding manager and, in my opinion, it is going to be very difficult to replace somebody of that stature."
Source: PA
Source: PA