McGhee saw his side dumped out of the League Cup by Dundee
Aberdeen manager Mark McGhee warned of team changes after the 3-2 Co-operative Insurance Cup defeat at Dundee.
Defensive lapses helped hand the First Division side a surprise victory.
And McGhee said: "I'm bitterly disappointed and what it does is leave an impression about certain people and about the squad going forward.
"I'll keep those thoughts to myself. But moments like this could be defining moments and certainly in some ways it is for me."
Dundee raced into a two-goal lead with goals from Eddie Malone and Craig Forsyth either side of half-time.
A double from substitute Michael Paton appeared to have pulled the Dons back from the brink of defeat, only for Leigh Griffiths to win it for Dundee in extra time.
McGhee has struggled to remould his squad since taking over from Jimmy Calderwood in the summer and the defeat at Dens Park comes only weeks after an embarrassing early exit from the Europa League kicked off his tenure.
Aberdeen went into the match on the back of four straight clean sheets but were guilty of some shambolic defending in Dundee.
Despite their record going into the tie, McGhee insists he was not surprised by the frailties exposed by their First Division hosts.
He added: "The winning goal was similar to the others in terms of there being a lapse in concentration. It was a diagonal ball and we got caught. It was poor, poor defending.
"One thing I've not got suckered into in the last few weeks has been people talking to me about us keeping four clean sheets.
"I've kept saying I wasn't getting carried away by that because I suspected they were down to a couple of great performances from Jamie Langfield, a great performance from Jerel Ifil on Saturday and a good performance at Falkirk from Andy Considine and Jerel.
"Suddenly we hadn't become a great defensive team.
"I always thought we were in danger of losing goals, even in those games."
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