The 1-0 defeat left Celtic bottom of Group H but still in with a chance of qualification to the knockout stage with two group games left.
"There's still a lot to play for in the group and it's still very tight," Lennon told reporters.
"What we've got to hope for is that Barcelona beat Ajax and we beat Milan and we go second in the group again."
Celtic are just two points behind second-placed Milan with leaders Barcelona already guaranteed a place in the last 16. The next matches are on Nov. 26.
"It may go down to the last games (on Dec. 11) and we're in the Nou Camp which is not the easiest place to go to but we have to improve, especially on our first-half performance if we want to progress."
Lennon assessed they were a lot better after the break, even though the game's only goal, from Ajax's Lasse Schone, came six minutes into the second half.
"The goal came at a time when we were just getting a foothold in the game and it was a poor goal from our point of view.
"Some players will have regrets about their performances because they weren't anywhere near their best."
- Sports & Recreation