Since then, Hearts have made their title push very clear with six wins and a draw to go third on the Scottish Premier League table, within striking distance of the Old Firm.
Jefferies admits there was plenty of soul searching done the week after the home loss to Killie but now he feels like it was the loss they needed.
"There were words in that Monday morning meeting which couldn't be repeated!" Jefferies said.
"But since then we have never looked back.
"We were fortunate our next game just happened to be the derby (with Hibernian) and we went into that with the right attitude and got the right result.
"So maybe Kilmarnock did us a little bit of a favour - and maybe we did them one as well because they got a big confidence lift from beating us the way they did."
And ahead of the Boxing Day clash between the two clubs at Rugby Park, Jefferies handed his old club some compliments.
"They are playing well and have rightly been given a lot of plaudits," he added.
"They are a club I strongly feel will be in the top six come the end of the season."