United currently sit fifth in the Premier League table, six points behind fourth-placed Manchester City.
In his weekly blog, Mata wrote: "The team was performing well in the previous weeks and it hurts not to have the consistency that you need to shorten the gap with the teams standing ahead of us in the table. Even more this weekend when they were facing each other.
"It's a message that we have repeated before this season, but the truth is that the only way to get to a better position is by working hard for the next game. There is no other way. Mourning is useless now; besides, there are still many games to play and many things can happen this season."
Meanwhile, reports have claimed that La Liga side Atletico Madrid are due to open talks with Mata regarding a summer transfer.