The Everton winger played with and under Lennon in Glasgow so is well-versed to know just how much of an impact his departure will make.
Lennon has ended a four-year tenure as Bhoys manager and former Parkhead ace McGeady is wondering who will be the man to step into the shoes of the three-time Scottish Premiership winner.
"It's a big blow," said McGeady.
"Now Celtic will obviously be looking to replace him with someone, but the big question is who do you replace him with?
"That's the first question, which candidates are out there? You start looking straight away.
"It will be a difficult job because Celtic is a massive club and success is a way of life, you have to go and win every game, you have to go and win every trophy you compete in, apart from the Champions League.
"If you are not winning two trophies a year, you are seen as a failure."
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