The Brazilian could only manage 20 minutes as a second-half substitute in the Reds' 2-2 draw with Aston Villa at Anfield, saying he heard a "click" in his knee.
The influential defensive midfielder has made 21 appearances this season, playing an important part in key wins over Manchester United and Tottenham Hotspur.
Speaking exclusively to FourFourTwo at a charity event arranged by official FA Cup sponsors Beko, Barnes admitted it is a blow to Liverpool boss Brendan Rodgers.
"It's always a blow when you lose players of Lucas' quality," the former England international told FFT.
However, Barnes is a fan of Allen and insists the young Welshman is more than capable of filling in for Lucas.
Allen moved to Anfield from Swansea for £15 million in August 2012 and has since made 49 appearances, finding the net twice.
"Joe Allen is a ready made replacement, he came with a big reputation from Swansea and he did well last year. He still has that potential," he said.
"He still has that quality, he'll be hungry to show that he can come in and fill those shoes. So I don't think that it will be too much of a miss."
John Barnes was speaking at the launch of Official FA Cup Supporter Beko’s Keepy-Uppy Challenge. To enter, click here – Beko will donate £1 per entry to Barnardo’s.
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