The Red Devils manager came under increasing pressure on Tuesday following a 0-0 draw with Spanish club Valencia at Old Trafford in the Champions League, prior to which club legend Paul Scholes had said that he was "embarrassing the club".
Fuel was poured on the fire by the 33-year-old, who liked an Instagram post criticising the manager after the draw.
"I'm not even surprised about the outcome of this game," the user posted. "I always look forward to our games but recently Mourinho has made watching us a punishment now. Something needs to change. It's time for Mourinho to go."
Valencia was spotted as having 'liked' the post and was forced to post a hasty apology online.
Posting on Twitter, he said: "Yesterday, I liked a post on Instagram without reading the text that accompanied the picture. These are not my views and I apologise for this. I am fully supportive of the manager and my teammates. We are all giving our everything to improve the results."
Yesterday, I liked a post on Instagram without reading the text that accompanied the picture. These are not my views and I apologise for this. I am fully supportive of the manager and my teammates. We are all giving our everything to improve the results. pic.twitter.com/bMEsrAwMkh— Antonio Valencia (@anto_v25) October 3, 2018
In the aftermath of the game, Valencia had pledged his support to the former Real Madrid, Chelsea and Inter boss amid reports of a falling out between the duo.
"No, I have never had any problem with anyone, thank God," Valencia said. "The decision [about the captaincy] is for the coach to make, now we need to see the positives. We played an important game and got a point, now we have a game against Newcastle and we need to fight.
"What I can say from the inside is that the lads are calm, working every day with a good attitude. We haven't got good results and that's why rumours arise."
The Ecuador international right-back played his fifth match of the season for the Old Trafford side on Tuesday. He had missed the opening couple of fixtures of the season due to a calf strain, meaning Paul Pogba took the captain's armband, and was also rested for last weekend's surprise 3-1 defeat at West Ham.