The reverse came just days after defeat at Crystal Palace.
Van Dijk said afterwards: "Obviously they got the penalty, they got a couple of opportunities basically through our mistakes in possession, losing the ball in unnecessary moments, a bit sloppy. Listen, you stand here with a loss and that's obviously the frustrating part, especially after the terrible performance we had over the weekend. So, take this on the chin and we have to recover quickly and be ready for Stamford Bridge.
"Obviously we started the season fantastically with all the wins that we had and now losing these two games doesn't feel nice. But I've been in the game quite a long time, so it's all about just keep working and head down. That's the main thing.
"Like I said, I hate losing and there shouldn't be panic, but there definitely should be improvement needed. I think it was a step-up in terms of the willingness, the hard work that we put in from the weekend, but that was not really too difficult because on the weekend it was just bad.
"But now we just have to convert chances, don't give a penalty away today and don't make silly mistakes at times. It's far too easy to say these things than to actually make it happen. But it's a process and we have to keep going."