Frank Lampard reckons Lionel Messi is the best player in the world at the moment and he fears what damage the Barcelona man will do to Chelsea.
The Blues will renew their Champions League rivalry with Barca after completing a bewildering 7-5 aggregate victory over Liverpool at Stamford Bridge.
Barcelona are favourites for the semi-final with the first leg at the Nou Camp in a fortnight with Messi, the competition's top scorer, the pick of their frightening array of talent.
The Argentina playmaker has been compared to Diego Maradona and Lampard is concerned how Chelsea will cope with his threat in the absence of Ashley Cole, whose suspension has left the club without a recognised left-back.
"Losing Ashley is a huge blow. He has been absolutely outstanding this season," he said.
"Lionel Messi is the best player in the world by some distance at the moment and will be on the right wing. If you wanted anyone to play against him it would be Ash.
"Unfortunately he got a yellow card but we have to try and keep together, find someone to come in and do a good job against him for the team."
Chelsea and Barcelona have met eight times in the Champions League - with four of those matches staged in 2006 - since the turn of the century.
So far the series is even with three wins apiece and two draws but Barcelona are flying this season, six points clear at the summit of La Liga and highly fancied to repeat their success of 1992 and 2006.
Lampard acknowledges Chelsea, who have conceded seven times in their last two matches following last night's enthralling 4-4 draw with Liverpool, must stop leaking goals if they are to prevail.
"We have to look back at the mistakes we make as a group in conceding goals. We have to be strong and we need to tighten up defensively," he said.
"If we are going to beat Barcelona we need to play at the level we achieved in the first leg against Liverpool when we won 3-1.
"It will be a very tough game because of the way they're playing, but we know what to expect, particularly in the away leg.
"Barcelona are probably favourites at the moment because of the way they're playing, but being favourites doesn't mean you go through.
"Barcelona are the form team in Europe, if not the world. They've been like that all season so there can't be a tougher match for us in this competition."
Lampard scored twice on a nailbiting night at Stamford Bridge, including the final goal that ultimately thwarted Liverpool's remarkable fightback.
"There were moments when I couldn't believe what was happening," said the England midfielder.
"We all had heart attacks on the pitch - as did both sets of staff and fans.
"It was real see-saw stuff. We were comfortable and then nearly out. It was a beautiful game in those terms.
"It was completely crazy. We didn't want it to be crazy - we wanted it to be much simpler than that.
"After half-time we showed great character and turned it round. We deserved to go through in the end."