Hibs had picked up only four points in their previous seven games before battling to their first victory over Celtic at Easter Road in four years.
Leigh Griffiths rounded Parkhead goalkeeper Fraser Forster to score the game's only goal from an acute angle after just eight minutes.
Irishman Fenlon 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.
"The win against Hearts a few weeks ago was a massive result and I think the players will take a lot of confidence from this result.
"It will give the players a lift (ahead of the Hearts game).
"I think over the last three or four weeks people have started to doubt us a little bit so we have answered a few critics, so we are delighted.
"We scored a very decent goal and probably could have scored again before half-time.
"It was a great finish, he still had a lot to do when he went round the keeper.
"Celtic put us under pressure with a lot of balls into our box but I don't think they carved us open or created too many chances."