Ambrose has just returned to the squad after a short summer break due to international commitments with Nigeria.
The Hoops have several injury concerns going into Wednesday's match with Adam Matthews, Charlie Mulgrew, Virgil van Dijk and Joe Ledley all at various stages of recovery.
But Lennon is ready to go with an experienced addition, and the Irishman is confident Ambrose will not let him down.
"Efe Ambrose is in contention," Lennon said. "He looks great, he looks in good condition. These guys play almost 12 months a year. He has had a couple of weeks off.
"He had a heavy schedule with the Confederations Cup, had a holiday, did a little bit of training in Nigeria and then joined us yesterday and he looks fine.
"It probably would have been ideal for him to do a little bit of conditioning work but looking at him yesterday he looks in fine fettle."
Meanwhile, Lennon has warned his team to maintain focus ahead of their first competitive fixture of the campaign.
"Anything can happen in football," Lennon said. "It is a dangerous game and it is one we are not taking lightly. We will have to be at our best.
"I know it is early and I know some of the players won't be at their best but we will have to play very well to get a result. Cliftonville will be highly motivated but the Champions League is so important to us as well."