O'Neil concedes the performance wasn't up to expectations.
He said, “I’ve got quite a lot out of the others, but I didn’t enjoy today. I thought it was flat and I didn’t think we got much out of the game. The way Vallecano played didn’t help us get much from it either because they were really direct, went 1-0 up quite early and let us have the ball, and we struggled to progress, which we might face sometimes, but it wasn’t like Premier League game today.
“But what I did get from it, was that we really struggled with large parts of it. We were obviously trying to win today’s game but trying to get a few people ready for the weekend against Arsenal, which will be a very different test.
“It was my least favourite game so far, and sometimes it can be a bit like that, the last game of pre-season, and you want it to be everything to get you prepared, and then they go 1-0 up early, it’s hot, and then the pitch gets slow, and we were not at our level quality-wise.
“I saw it in training as well the other day, since we landed back from America maybe the level has dropped slightly, whether there’s some tiredness because there was a lot of travel, but that’s out of our system now and we need to get back to work on Monday.
“The lads have seen the difference of when we are at our level against West Ham and Leipzig and even Crystal Palace at times, and then today when we dropped well below it. It’s been a frustrating day and I didn’t enjoy it, but there’s still stuff that we can learn from it which will be important for us.”
On preseason overall, O'Neil also said: “I have enjoyed preseason, apart from today. But my staff are keen for me to not just see all the negatives, and they were in there trying to remind me it’s been a good pre-season and today was below our level, but over the course of the last five weeks, there’s been some fantastic work and some really good performances, but there was a lack of quality today when we got into good areas.
“We missed passes and we missed crosses, made some real strange decisions, but over the course of the five weeks there’s been some really good stuff. The new lads have gelled well, there’s a good feeling in the group, and maybe today was a good reality check that we can’t just go out there and expect it to be fantastic all the time.
“There’s a lot of work that goes into it and we need to make sure we get to our level. I’m looking forwards to watching it back because there’s lots of things in there which I think can help us with. But it’s been a positive pre-season. We haven’t conceded too many goals, we’ve looked an attacking threat, we were just a little bit slow and off it today.”