As solid as a player can get. One of his best attributes is his ability to use his strength and pace to get out of tight situations. It's as clear as daylight that he's been instructed not to commit forward too much, and allow the likes of Mata and Herrera to be more creative.
He's fine as a back up player. He's versatility and willing to give 100% whenever called upon, even if that means not playing every week. You need people like that in your squad. If he plays every week at right back then eventually we're gonna see mistakes from him that cost us, the one against Arsenal being a prime example. Further forward he can still be effective, just not often enough to be playing there week in week out anymore, unfortunately Hopefully we bring in a new right back and winger next season so he can be used more sparingly. Don't see any point in selling him, we'd not get that much and he's a good player.
Against Liverpool, Valencia did struggle quite a bit with Sturridge's movement. He was often unsure about whether to follow Sturridge around, stay inside, or stay outside. However, given that Coutinho and Moreno barely got a sniff on their left side thanks to our midfield and Mata doing a great job of shutting them down, the supply to Sturridge was pretty much shut off, particularly when they went down to 10 men. Thus, Valencia didn't have as much trouble against Sturridge as he did against Sanchez, for example.
On the ball, though, he was very reliable and calm. For someone so right-footed, he really is excellent at keeping the ball, controlling it, maintaining possession, and passing it into space. I was also very happy with how disciplined he was, overall. His reliability on the ball is a big reason as to why he starts regularly.
I still don't think he can be a regular right back as he will be exposed against good-quality teams, but he's quite a reliable option to have for many matches.