Lionel Messi is facing six weeks on the sidelines after sustaining a thigh injury during Barcelona's Champions League victory over Celtic.

The 20-year-old left the pitch in tears late in the first half of his side's 1-0 win with a thigh muscle tear in his left leg, the fourth time the Argentine has suffered this type of injury in the last three seasons.

The fact that Messi's game relies so much on his explosive acceleration has heightened concerns about the latest injury.

He was out for five weeks earlier this season with a similar problem in the same leg.

"It is a very sad moment for the player," said Barca coach Frank Rijkaard.

Rijkaard had come under fire from the local media for leaving Messi out of the starting line-up during Saturday's 4-2 defeat at Atletico Madrid and Barca captain Carles Puyol said they were partly to blame for the injury.

"Messi's injury is a lesson to everyone," he said. "You pressured for him to play against Celtic and now look what has happened. What you have to do is show more respect for the medical staff and the coach.

"Rijkaard took the decision to rest Leo [against Atletico] at the Calderon because there was a risk of injury. Now I suppose you will be criticising the medical staff because he is injured."

Midfielder Xavi, who scored the goal that completed Barca's 4-2 aggregate win over the Scots, said the loss of Messi was a serious blow to the club's prospects in both the league and Europe.

"We paid a very high price for this victory," he said. "Leo was in fantastic form. It is a real shame."

Reuters