Saracens director of rugby Mark McCall admitted his England number eight Billy Vunipola had "stuff in his head" over the 24 hours before the Gallagher Premiership high-flyers suffered a surprise 23-21 defeat to Bristol at Ashton Gate.
Tongan-born Vunipola was booed every time he touched the ball after entering the fray as a replacement after 51 minutes of the game.
Vunipola faces a grilling by RFU bosses over his stated support for Australian Israel Folau who has been criticised over homophobic posts on social media, with his national union stating they intended to terminate his contract.
The match was tight throughout and came down to a penalty, conceded at a ruck 30 metres out, which Bristol replacement fly-half Ian Madigan slotted to earn his relegation-threatened side their seventh league win this season.
Bristol had scored tries via flankers Steve Luatua and Dan Thomas, with fly-half Callum Sheedy adding two penalties and two conversions in addition to Madigan's kick.
Saracens' points came via tries from scrum-half Tom Whitley, centre Marcelo Bosch and replacement prop Ralph Adams-Hale, with full-back Alex Goode booting all three conversions.
But the post-match discussion with McCall was dominated by Vunipola, who had controversially defended Folau.
Vunipola's Instagram posts are now the subject of a Rugby Football Union investigation and are also being looked into by the club.
McCall said of Vunipola's mindset going into the match: "He is a human being and he is also keen to do well for his team-mates. He is a great club man. I thought he did well when he came on in fairness.
"But he is a human being and I am sure there is stuff going on in his head. The club have made a clear statement and we have not sat down with him yet. I will talk to him myself.
"The decision to play him (Vunipola) from the bench was made before what happened, happened."
McCall said of the booing during the game: "I am not sure if it is surprising. It was unexpected. I don't want to get dragged into anything. My opinion is not that relevant."
It all comes a week before Saracens tackle Munster in a huge Heineken Champions Cup quarter-final.
McCall said of that game: "It is seven days away and we have to look at it. We will make rugby decisions ahead of our selection."
The booing of Vunipola overshadowed a crucial win for Bristol at the bottom of the table as they battle against relegation.
Head coach Pat Lam said: "To play a team like Saracens, you have to play a certain way to be successful.
"Fair play to the boys, we were disappointed after our game at Twickenham (against Bath last week) which was another game we contributed to the loss.
"There were some big changes we needed to make after that and really emphasise why we were doing it.
"Bar a couple of mistakes, they implemented our plan well and we got the win out of it.
"We want to be in the top six in the league but that is me being impatient.
"You have to go through pain to get there and so the way we look at it, there are three more games."
