United made it four games without a win in all competitions after another limp performance in their second Champions League match of the season.
There was some improvement on last Saturday's display in their 3-1 loss to West Ham, but Valencia were not unduly troubled by the United attack and could have taken all three points had they made the most of opportunities on the break.
Speaking afterwards, Mourinho seemed to warn United fans not to expect much success from 2018-19 and claimed he could not be honest in his assessment for fear of further criticism of his methods.
"I think there are things that is difficult for me to say and if I say something funny, I'm leaving myself... it would be another reason to criticise me and I think you have enough," he told a news conference.
"So, I prefer just to say that when last season we finished second and we finished runners-up in the FA Cup, we won our group in the Champions League. I thought it was a fantastic season. I knew why."
Earlier, Mourinho appeared to suggest it would be difficult for United to improve upon their achievements of last season, when they finished second in the Premier League to Manchester City but failed to win a trophy.
He told BT Sport : "I told in pre-season that last season was phenomenal. To finish second, reach the FA Cup final, win the group in the Champions League... I said it was a very, very good season and I don't say more than that."
4 - Jose Mourinho has gone four consecutive home games in all competitions without a win for the first time in his managerial career. Frustration. pic.twitter.com/hHV9kQ6fSg— OptaJoe (@OptaJoe) October 2, 2018
Mourinho felt the draw was a fair result but again seemed to question his players' quality by claiming his side lack defenders capable of building attacks.
"Frustrating, because it's not the result we want," he continued. "There were some improvements in our performance. [We knew] if we don't perform at least at an acceptable level, we would lose and defeat would leave us in a difficult situation.
"In such a difficult group, if you cannot win, you don't lose. We were the team that tried more, that had probably a couple of big chances to win, but I accept the result as a fair result.
"The players, they tried, they raised the level of their effort, they raised the level of their intensity. We tried to play but in some crucial phases we didn't have the technical quality to build from the back.
"We knew we would not create 20 chances. Our attacking players are not in their best moment of confidence and individual performance level, so we thought with three or four chances we would score and win the game, which we didn't.
"It's not a bad result but it's not a good result. We have Newcastle before the league stops [for the international break] and after that we have Chelsea and Juventus in three days. It would be really good for the team if we can beat Newcastle."