Fumbling a loss away to Hibernian on Sunday raised questions over whether Celtic goalkeeper Artur Boruc was mentally focused to compete at the top level.
If there were any doubts before tonight's CIS Cup tie against Dundee they were banished in a terrific display as, thanks to goals from Scott McDonald and Jan Vennegoor of Hesselink they defeated a gritty Dundee side 2-1.
For the first 20 minutes Celtic dominated early possession, but after that the game was brought to life with Dundee being denied by the woodwork twice in the space of 30 seconds.
Despite these positive results in their attacking third it would be defensive lapses that would result in the visitors taking the lead after 27 minutes.
A cross by Derek Riordan from the right side was met by an unmarked McDonald who happily headed the ball past Craig Samson for a 1-0 lead.
They should have made it 2-0 early in the second half but a penalty by Derek Riordan was saved by Samson.
In the 60th minute amends were made when a cut-back pass by McDonald was struck by an unmarked Vennegoor of Hesselink to make the second goal count.
However, Dundee would continue to graft away and after several attempts they scored from a shot by Kevin McDonald which proved to be a glimmer of hope, but one that came too late.