The Irishman said: "That's probably the biggest win since we've been here.
"To beat Celtic is a fantastic achievement, after the run they have been on of six or seven wins and after qualifying for the last 16 of the Champions League. It is a big result."
Now the Hibs boss is anxious to keep Leigh Griffiths, who netted the winner on Saturday, and fellow loan stars Ryan McGivern and Jorge Claros beyond next month.
"We will probably sit down soon," said Fenlon. "We are anxious to keep all three.
"Jorge is a good player and we want to keep him.
"Ryan had a tight hamstring (which kept him out) but he has been a fantastic signing, while Leigh has scored a lot of goals.
"We can only do so much. The board is criticised when it comes to keeping players but we are doing as much as we can in relation to Leigh and the other two.
"It probably will not be our decision, it will be the clubs to whom the players are contracted."
Fenlon is hopeful of having McGivern back for the trip to face Hearts at Tynecastle on Thursday.