Erik ten Hag made six changes from the side that beat Arsenal on Sunday (4 September). Cristiano Ronaldo, Harry Maguire and Victor Lindelof were among those given the opportunity.
But the Red Devils put on a meek display at Old Trafford. A Brais Mendez penalty was enough to give the Spanish side the win on matchday one.
Another player handed a start by Ten Hag was Casemiro. He played the full 90 minutes for the first time since his big-money move from Real Madrid in the summer.
The defensive midfielder was a key member of the side that won five Champions League titles with Los Blancos in defensive midfield. But due to the improved performances of Christian Eriksen and Scott McTominay, the Brazil international has been forced to wait for his chance.
Casemiro was handed his opportunity this evening against Sociedad, but failed to impress on his full debut. The midfielder was slow on the ball and looked off the pace in Ten Hag's team.
After the full-time whistle, Manchester United fans took to Twitter to express their concern over Casemiro's sub-par performance:
Louis Saha warns Casemiro that he may have to settle for a place on the Manchester United bench
Many expected the former Real Madrid star to go straight into Ten Hag's first team, but McTominay's excellent form has quashed those plans.
Former Manchester United striker Saha believes Casemiro will have to be patient, as the French pundit told Compare.bet:
"He (McTominay) has youth on his side and he’s done well. I do think that Casemiro’s experience will help him and the squad. I’m sure about that.
"It’s a very long season and there will be injuries and resting, players will dip in and out of form, so everyone in the squad has to be ready. And this squad for Manchester United is amazing."
Saha was also full of praise for McTominay's midfield partner Eriksen, as he further added:
"I think it adds directness. It’s a different formula and it allows Bruno Fernandes to be a bit more deadly going forward with more freedom. He’s a very very smart player. He has great quality on the ball and dictates the rhythm of the team and the midfield with his passing range. He has legs too, he can run!
"You see him pressing high and a minute later making a run down the middle to assist Rashford. For anyone to defend against that, where he’s a bit of a box-to-box player, but also a master passer, it’s tough. He’s a very very special player."