Hoops defender Mikael Lustig was stretchered off the pitch after United defender Gavin Gunning clattered into him in an aerial challenge.
Lennon said: "I thought the challenge on Mikael was reckless to say the least.
"There is no fracture thankfully, it might be ligament damage. It is a relief because he is a top player."
Gunning had escaped punishment earlier in the match for kicking out at Virgil van Dijk, while McNamara believes Celtic skipper Scott Brown should have been punished for a tackle on Stuart Armstrong.
United manager McNamara defended Gunning when asked for his view on the Lustig incident.
"I think he went for the ball and they came together," McNamara said.
"I think a reckless challenge would be Scott Brown on Stuart Armstrong, I would say that was reckless."