The second half was tense with Celtic's Fraser Forster saving from Steven Whittaker and Rangers keeper Neil Alexander denying Ledley.
And Celtic were in front by half-time as Rennes failed to cope with a long ball from keeper Fraser Forster.
"John's had the ghost of Fraser Forster hanging over him for months on end and it's not an easy thing, " he said.
The former Colchester manager has already made his first signing for Norwich, bringing in Newcastle goalkeeper Fraser Forster on an emergency loan.
Scott Robertson picked up the loose ball, but the United midfielder's attempted chip over Fraser Forster was saved on the line by the goalkeeper.
The striker smashed the spot-kick beyond the outstretched arm of Fraser Forster after Foran had been felled by Loovens following Russell Duncan's through ball.
As half-time approached, Peter McDonald looped a header towards Celtic keeper Fraser Forster that cleared the bar, then a Grainger corner failed to test the visitors' defence.
Kevin Kyle should have scored with an 18th-minute header for Hearts when he was left unmarked at a corner, but goalkeeper Fraser Forster saved well at the back post.
Mensing then forced Celtic goalkeeper Fraser Forster to get down and save a weak shot after a neat one-two brought the Hamilton defender bearing down on the Celtic goal.
Lennon thought his side started sluggishly, but he was generally delighted and picked out forward Shaun Maloney, winger Paddy McCourt, left-back Emilio Izaguirre and goalkeeper Fraser Forster for special mention.
The 18-year-old Watt, making his first appearance in Europe's top club competition, made it 2-0 after veteran midfielder Xavi failed to cut out a long clearance by Celtic keeper Fraser Forster.
It took a moment of magic from Riordan to bring new life to Hibs' challenge, the forward sending a superb lob from the edge of the penalty box over towering Celtic goalkeeper Fraser Forster and into the net after 54 minutes.
Chris Maguire was first to have an effort for the visitors after Darren Mackie won the ball in midfield, but the young striker took too long before shooting allowing Celtic keeper Fraser Forster to narrow the angle and put the ball out for a corner.
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