Lamela and Spurs team-mates Giovani Lo Celso and Sergio Reguilon broke coronavirus rules by attending a Christmas party with at least nine other adults, including West Ham’s Manuel Lanzini.
It is unclear exactly when the party took place but the three Spurs players have since been tested more than once, with Lamela missing Saturday's 3-0 win over Leeds, while Reguilon was an unused substitute.
Lo Celso had previously been ruled out of Spurs’ festive programme by Mourinho through injury.
Lamela recovered from an Achilles problem to feature in Spurs' fixtures either side of Christmas against Stoke on December 23 and Wolves on December 27 but, asked if he would play in the one-off tie against the Bees in north London, Mourinho said: "He's not available."
I want to apologise for a decision I made over Christmas which I deeply regret. On reflection I understand the seriousness of my actions and the impact it has on others.
I am truly thankful to everyone working hard to keep us safe and I feel ashamed knowing I’ve let people down.
— Erik Manuel Lamela (@ErikLamela) January 2, 2021
Pushed on the reason for the winger’s absence, Mourinho added: "It’s not for me to tell you [why]. I just tell you that against Brentford he cannot play."
Spurs 'strongly condemned’ the trio's behaviour and said the matter would be ‘dealt with internally’, while Mourinho afterwards said he was "disappointed" – not least because he had gifted Reguilon a suckling pig for Christmas, believing the Spaniard would be spending the period alone.
Argentina internationals Lamela, Lo Celso and Lanzini have all since apologised on social media.
Lanzini came off the bench in West Ham’s win at Everton on Friday, with the Hammers later saying the forward had been ‘reminded of his responsibilities’.
Mourinho can also expect to be without Gareth Bale, who injured his calf in the last round at Stoke, against championship side Brentford, while Lo Celso is set to miss the rest of January with a hamstring problem.