Bangura was on the pitch at half-time drumming up support for his appeal against the British Home Office’s decision to deport him back to Sierra Leone.
It did little to inspire a lacklustre Watford, though, and David Norris snatched an 89th-minute winner for Argyle.
Boothroyd said: “It would be a great opportunity to blame Al or injuries, but the truth is we had chances and didn’t take them and they gave us enough warnings and we didn’t heed them.”
Meanwhile, Hornets assistant manager Keith Burkinshaw parted company with the Hertfordshire club yesterday for family reasons.
Bristol City are third after their 1-0 home win over 10-man Cardiff thanks to a Marvin Elliott header.
Robins manager Gary Johnson admitted his side had found it difficult against the 10 men.
“Sometimes you have to give credit to the opposition and I thought Cardiff reorganised well and closed us down quickly,” he said.
QPR remain bottom despite a 0-0 draw against Wolves at Loftus Road.
Stephen Elliott had a great chance to win it for the visitors but missed from close range, and Rangers manager Luigi De Canio admits if the players continue to impress him he will not need to be as active in the January transfer window.
“There is a lot of talk about who we will sign in January, but there are lot of games before then and the league table at the moment is not good, so we must focus on that.”
Coventry manager Iain Dowie will be giving new chairman Ray Ranson his shopping list when the pair meet for the first time next week.
Dowie said: “I always have a shopping list ready. There are one or two areas we would like to look at and if we want to take this club forward we have to do that.
“The new chairman wants to sit down with me next week and he has said he wants to invest from the academy upwards and strengthen the first team.”
In the other matches, a late goal from Irish teenager Shaun Scannell earned Crystal Palace a 2-1 win over Sheffield Wednesday, Stoke won 3-2 at Blackpool and Preston won a thrilling Lancashire derby 3-2 at Burnley.
Colchester and Norwich drew 1-1, Hull beat Leicester 2-0 and Ipswich made it 10 wins from 11 home league games by beating Scunthorpe 3-2.
Sheffield United overcame Barnsley 1-0 in a South Yorkshire derby at Bramall Lane.