United's staff are said to have been wowed by the youngster since making his debut in December.
United, currently second in the top-flight, trail neighbours City by eight points after 11 games.
"We return to training with the mindset on the Newcastle match and the importance of the months that lie ahead, as they come loaded with games, especially during the Christmas period", Mata wrote on his blog on the club's website. He also attended the MTV European Music Awards in London on Sunday night with his brothers Florentin and Mathias.
But, according to Marca, the Spanish club will be forced to sell Soler next summer if they don't qualify for the Champions League this season.
Mourinho is committed to United for now.
United could let Mata leave the club in January as they look to cash in on a player whose contract expires at the end of the campaign.