Celtic were beaten 2-0 at AC Milan on Wednesday night in their Group H opener, and mighty Barca are next up for Lennon's men at Parkhead.
Barcelona thumped Ajax 4-0 to get their campaign up and running with a bang, with Lionel Messi scoring a hat-trick, and Lennon admits watching that game has given him cause for concern.
"It was very impressive and obviously concerning as well, considering we play them next," he said.
"Barcelona will be hurting from last year in the Champions League, particularly the semi-final, and they will come back reinvigorated.
"When you have a player the calibre of Messi he can make any team better, and allied to Neymar, who is in their ranks as well, and the attacking options they already have, then they are obviously in superb form because Ajax are a very good team as well.
"It takes a lot to surprise players of that experience and quality, who have been playing at the highest level for the last seven, eight, nine, 10 years.
"They have done everything; World Cups, Euro Championships, Champions Leagues so it will be very difficult for me to come up with something that they haven't seen before."
Lennon, looking forward to the Scottish Premiership game against St Johnstone at Parkhead on Saturday, reiterated how pleased he was with his side's performance in the San Siro.
"Our first 15 to 20 minutes after the break was superb, the football we played and the chances we created," he said.
"It gives you a lot of encouragement, obviously."
Turning to the visit of Tommy Wright's St Johnstone side, Lennon said: "I know Tommy very well personally and I am delighted for him that he has got the opportunity to take over from Steve Lomas.
"They have been very consistent over the last couple of seasons. They have a very good team ethic, they work hard for each other and they have strikers who can upset us, so it is a good game to come back to."