The midfielder has been battling all season with a hip complaint, while only playing intermittently and usually in the big games.
However, he had to succumb to the pain in the second half when he was replaced by Kris Commons.
Parkhead boss Lennon said: "Scott Brown has run his course, we may have to book him in for surgery.
"We are looking at eight to 10 weeks. He has been pushing himself to the limit and we can't keep asking him to put himself through the amount of distress that obviously he was doing towards the end of his time on the pitch.
"He has been a huge player for us this season.
"I think it is time to get the problem addressed and, hopefully, he will have something to look forward to in the new year."
The boss was alluding to the fact that Celtic's bid to win through to the knockout stages of the Champions League will go to their last Group G fixture.
Ola John gave the home side the lead in the seventh minute before Georgios Samaras levelled in 32 minutes from a Charlie Mulgrew corner.
Defender Ezequiel Garay restored Benfica's lead in the 72nd minute to ultimately, and deservedly, leave both sides on seven points after five games.
The SPL champions, guaranteed a Europa League spot after Barcelona beat Spartak 3-0 in Moscow earlier in the evening, will go through if, in their home game against the Russian club, they better the Portuguese side's result in Barcelona.
Lennon admits his last-16 hopes are in the balance.
He said "I think Benfica are capable of getting a win in Barcelona, for they are an excellent side. But we have to make them do that. I don't know what kind of mindset Spartak will be in.
"They may well want to spoil our party, so we have to be totally focused.
"What we have to do is win at home and make Benfica win in the Nou Camp, which is no easy feat. The onus is on us to do that.
"The squad is stretched, we hope to have a few more players back for that game. No-one gave us a prayer coming in to this competition.
"These players have been exceptional, they have made a name for themselves and we are still in with a good chance of qualifying."
The manager had few complaints about the result. He said: "I am really disappointed.
"I thought there was a lack of composure to our play at times, a bit of nervousness which is understandable.
"Our passing was poor and we weren't decisive enough and it was a poor second goal to lose. We weren't at our best, but you have to credit Benfica."
Benfica coach Jorge Jesus said: "It was a great win for us. We didn't deserve to be drawing at half-time, they scored with a set-piece, but we played well.
"We have shown we are more of a team than Celtic. Now it all goes down to the last matches, but we are still in with a chance."