Skipper Scott Brown faces two months on the sidelines as he undergoes hip surgery while the highly rated Victor Wanyama sits the game out through suspension.
Celtic must get a better result at home to Spartak than Benfica achieve away to Barcelona if they are to progress to the last 16 of the Champions League, with the consolation of a Europa League spot to fall back on if they come up short.
Brown has been used sparingly all season as he nurses a degenerative hip injury, but after lasting just 64 minutes last night his manager admitted he can't keep asking his skipper to play through the pain.
"We're looking at surgery now," Lennon stated. "I think that he's given us everything that he could in every game.
"It's really tough to keep asking him to push himself when he's in a lot of discomfort and distress.
"We might have to shut him down now and have the surgery. We'll assess it over the next couple of days but the signs aren't good."
Wanyama put in another towering performance last night while most of his team-mates failed to reach the heights they showed in two matches against Barcelona.
An 85th-minute booking for the Kenyan completed a miserable night for Celtic with Lennon's despair at the upcoming suspension obvious to the millions watching on television.
Following the euphoria of beating Barcelona last night's defeat was a dose of reality with Lennon giving an honest assessment of his side's shortcomings.
"I thought there was a lack of composure to our play at times," he admitted straight after the final whistle. "There was a little bit of nervousness about us, which was understandable for a young side.
"Once we got the goal, we showed a little bit more belief but, in the second half, we went back into our shell a little bit. We decided to change it to three in midfield, just to try and get more of the ball but our passing was poor at times tonight - our first touch let us down.
"We were not decisive enough in our overall performance. Against a side as good as Benfica, if you keep giving the ball away, eventually the pressure will tell.
"The second goal was a poor one to lose. We cleared the box at a corner, but didn't pick up the ball as it came back in. Garay was left with a free shot at goal. We had a couple of really good chances at the end to get a draw, but, again, it really summed up our night."
Ola John gave the home side the lead in the seventh minute before Georgios Samaras levelled in the 32nd minute from a Charlie Mulgrew corner.
Defender Ezequiel Garay restored Benfica's lead in the 72nd minute to leave both sides on seven points after five games.
Benfica coach Jorge Jesus will take succour from Celtic's win over Barcelona when he takes his side to Spain.
He said: "Now we are playing on the biggest stage in the world against the biggest team in the world. But we have shown we are more of a team than Celtic and they beat Barcelona so nothing is impossible."