The Brazilian, who joined the Reds from Hoffenheim for a reported £29m fee, has struggled to recreate the form that he showed during his side's 4-1 rout over Manchester City last month.
In that game, Firmino linked up well with his fellow Brazilian Philippe Coutinho, but the playmaker has been absent from Liverpool's last four matches with a hamstring injury.
On Sunday, Firmino struggled to make an impact in Liverpool's 2-0 defeat at St James' Park.
"Roberto Firmino is playing as if something is wrong, mentally and physically," Aldridge wrote in the Liverpool Echo. "He's been absolutely redundant the last two games after such a great display at Man City. It can't just be the fact his mate Coutinho has been missing.
"Firmino was the missing link and things improved when Adam Lallana, who played well, came on. It was like chalk and cheese.
"The Brazilian also pulled out of a challenge that was 55-45 in his favour. Liverpool fans won't accept that. I've been going for 50 years and I know it's unacceptable. If the ball is there to be won you win it. He's going to have to learn to get stuck in."
The Merseyside outfit are currently eighth in the Premier League table, six points off fourth spot.