Celtic beat Barcelona 2-1 at Parkhead in the group stages last season on their way to the last-16 but the Northern Irishman believes Barca, who were knocked out of the semi-finals last season by eventual winners Bayern Munich, will be a different prospect this time.
Asked about the midweek result in the Nou Camp, Lennon said: "It was very impressive and obviously concerning as well, considering we play them next.
"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."
It was the spurning of those opportunities in Milan which Efe Ambrose highlighted at the press conference at Celtic Park.
The Hoops defender said: "We have to take our chances. We created them but we never scored so it is something we should work on."
Lennon, though, highlighted the reality of Celtic's financial situation when asked about the lack of a Champions League goal scorer.
"We have lacked a Champions League goal scorer since I took over, we haven't had one probably since Henrik Larsson.
"They cost a lot of money and they are hard to find.
"What do you class as a Champions League goal scorer? Van Persie? Well, they cost B£20million.
"So yes, of course we are going to lack a Champions League goal scorer.
"Messi, Ronaldo, Shevchenko; they are proven Champions League goal scorers and you just don't buy them."
However, the former Celtic skipper believes that striker Anthony Stokes, who was making his Champions League debut in Italy, can make an impact in Europe's top club competition.
"I hope he does score at that level and I think he is capable of doing it," he said.
"He showed the other night that he is capable of creating chances on his own.
"He was very unlucky with a free-kick that hit the crossbar and he was pretty close to scoring before the break as well.
" I thought he gave AC Milan all sorts of problems, so for his debut at that level, he can be very pleased with himself.
"I am not surprised with it. He has been a little but ill-disciplined in his past but certainly since he came back from his injury, he has shown a high level of consistency with his performances and I am really pleased with him."
Lennon, though, admits a new deal for the former Sunderland player is still in limbo and that he is the "wrong man" to ask about the hold-up.
"I have spoken to him on a couple of occasions but we are not any closer," he said.
"I don't deal with Anthony on the money side of things, that's between the board and his agent."
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."
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