With qualification at the head of Group G already assured, Barcelona rested numerous first-teamers for the match, which the visitors dominated in the first half. But Messi came off the bench in the 58th minute. He twisted his knee trying to beat Benfica goalkeeper Artur in the 85th minute and looked in considerable pain.
The extent of the injury was unclear in the immediate aftermath of the final whistle. "Messi has a bruise on the outside of his left knee,'' Barcelona tweeted. "He'll undergo further tests to determine the exact extent of the injury."
"It's a bruise which doctors have been having a look at,'' Barcelona coach Tito Vilanova said, according to The Associated Press. "We now have to wait for the results of tests, but the feeling is that it isn't more serious than a knock.''
The latest updates from The AP can be found
The Argentine superstar is one goal short of equaling the German great Gerd Mueller's all-time record of 85 goals in a calendar year, which was set in 1972. Since the Spanish league takes a brief break around Christmas, Messi has only four matches left to equal or break Mueller's mark. Even a minor injury could put that bid in serious jeopardy.
Celtic joined Barcelona in the next round as runner-up ahead of Benfica. Celtic's 2-1 victory over Barcelona in Glasgow last month was a magnificent occasion and a triumph for effort above talent, will power before technique. Thanks to an 81st-minute penalty from Kris Commons that gave Neil Lennon's men a 2-1 win over Spartak Moscow at Celtic Park, that glorious night will not be the only memory to treasure from the club's exploits in Group G.
Gary Hooper's excellent low first-time strike midway through the first half from the edge of the box had given the Scots the lead, but, energized by the exit of their unpopular manager, Unai Emery, Spartak equalized through Ari's lovely chip over goalkeeper Fraser Forster shortly before halftime.
When Marek Suchy bundled over Georgios Samaras in the area, Commons stepped up and held his nerve to spark huge, and well-deserved, celebrations.
Fast forward seven months from that astonishing night in May and Roberto di Matteo, the title-winning manager, is already gone. And now the club has become the first Champions League winner to fail to reach the knockout stage in the next season.
A four-time semifinalist, Chelsea had never gone out in the group stage in 10 previous campaigns. Yet it is off to the purgatory of the Europa League after a 6-1 win over Nordsjaelland in Group E.
The thrashing was rendered irrelevant because Juventus beat Shakhtar Donetsk 1-0, scotching rumors of a pact between the clubs to play out a convenient tie that would have doomed the Londoners. Only a Shakhtar win would have helped Chelsea.
Any positives? Well, with only 21 goals since arriving at Stamford Bridge from Liverpool for an $80 million transfer fee in January last year, Fernando Torres is averaging just over five goals per Chelsea manager. But after one goal in the past two months, he might have had a hat trick Wednesday.
The game was more wild than anyone could have predicted. After 30 minutes of Chelsea dominance, Nordsjaelland had the chance to stun the home crowd when a penalty was awarded against Gary Cahill for a handball that was outside the area. Petr Cech, whose penalty saves were so critical against Bayern, came up big again, guessing correctly to stop Nikolai Stokholm.
Three minutes later at the other end, Mikkel Beckmann was penalized for handling in the area, but goalkeeper Jesper Hansen stopped Eden Hazard, whose run-up and attempt was so tentative, it seemed to symbolize Chelsea's lack of self-belief.
Yet only another four minutes elapsed before Dutch referee Bas Nijhuis' questionable interpretation of deliberate handball gave Chelsea another go. David Luiz assumed penalty duties and smashed the ball into the net.
In first-half injury time, Torres scored on the rebound after the goalkeeper saved his shot. Yet, 20 seconds into the second half, Joshua John pulled one back with a fine finish. It was an isolated incident.
A superb looping header by Cahill from a free kick in the 50th minute restored the two-goal advantage. Then Torres tapped in from close range and had chances to score again. Juan Mata added a fifth, and Oscar claimed the sixth.
Quite a way for Rafael Benitez to win his first game as Di Matteo's successor at the fourth attempt: comprehensive yet futile. Chelsea must feel as crushed as Nordsjaelland.
The holders' exit comes a day after Manchester City, the English champion, went out of Group D in fourth place. So, only Manchester United and Arsenal remain.
Last year, only Arsenal and Chelsea went beyond the group phase. Arsenal has qualified again from its group, as usual, but has never won the title, and it is far-fetched to imagine that will happen in this campaign.
As recently as 2008-09, three English clubs reached the semifinals. And the story of the Premier League since has been of ever-growing investment by billionaires and soaring TV revenues. So there is no financial explanation for this on-field decline.
How to rationalize it, then? Are leading English clubs worn down and distracted by the demands of an ever-more competitive Premier League? Has there been a raising of standards in Europe, especially a resurgent Italy, Germany and Spain? Or is it simply that success is cyclical?
Tell it to Barcelona, or Arsenal, or Manchester United, which lost at home to Cluj after resting players for the second successive match. Manager Sir Alex Ferguson used Wayne Rooney, Javier Hernandez and Danny Welbeck -- but also Alexander Buttner, Nick Powell and Scott Wootton.
United had already qualified as Group H winners, but there was plenty riding on the match for Cluj, which began the night in third but level on points with second-place Galatasaray. The Turkish side ultimately went through in second place thanks to a late goal from Aydin Yilmaz that saw it beat Braga -- but for a long time, it seemed as if Cluj would profit from United's indifference.
Still, given its notoriously lenient punishments for such serious matters as racism, European soccer's governing body is hardly going to get sweaty under the collar about a manager shuffling his pack.
United conceded once, which is modest by its recent standards, yet this time the team's attacking prowess did not rush to its rescue.
Captain Nemanja Vidic was not fit enough to return after his knee surgery in September, Ferguson also ruling him out for Sunday's Manchester derby. Without the Serbian defender, United could field pretty much field any combination at the back, and it would look vulnerable.
Cluj won 1-0 at Old Trafford courtesy of a fabulous 25-yard strike by Luis Alberto in the 56th minute, but no one closed down the midfielder as he charged toward goal, making the most of United's indefensible defense.
A healthy spread of countries have qualified for the knockout phase: Spain (four clubs), Germany (three), Italy (two), England (two), Ukraine, Turkey, Scotland, France and Portugal.
Back in 2008-09, Spain and England made up half the members of the last sixteen: Spain (four), England (four), Italy (three), Portugal (two), France, Germany and Greece.