Celtic were two up inside 21 minutes in Dingwall but lost 3-2 after Steffen Wohlfarth headed an injury-time winner for the hosts.
Victor Wanyama, Emilio Izaguirre, Adam Matthews and Georgios Samaras had all picked up injuries in Wednesday's Champions League defeat by Juventus in Turin while James Forrest was also absent, but Lennon admitted there was no excuse for his team's capitulation.
Lennon, whose side remain 15 points clear despite consecutive Clydesdale Bank Premier League defeats, said: "I knew it was going to be tough when we came here looking at the pitch and conditions. But we started brightly and you have got to take your chances.
"It should have been 4-2 at half-time, we had clear-cut chances, which on a pitch like this is difficult to do. And the game became a fight in the second half, which you expect it to.
"We lacked men in some areas. And I'm going to have to get some men in."
Lennon, whose team host Aberdeen on Saturday, added: "I'll make changes. I will hopefully have Samaras and Izaguirre back next week, we will see how Victor is.
"It gives us a week to recuperate. We have had a long week. We had a four-hour drive on Friday after coming back from Italy on Thursday.
"So there were mitigating circumstances for a bit of lethargy but what you can't lack is character and I thought we lacked that in a lot of departments. I'm pretty angry about that."