I don't think Silva had a particularly poor game, he just played a quieter part. Which I believe was a tactical decision.
I've noticed since last season, we've always tended to play a bit of a lopsided game. That is, we focus our efforts on one side of the pitch more. Normally, when Sané and Sterling start, we focus on the left. I think that's so Sterling can get into more advanced positions later in attacks. But an element of that game means that one of the CMs, normally Silva, plays much higher up the pitch and looks to consistently position themselves in between the lines and link up with the fullback and the winger on that side. Meanwhile, the other CM, normally KDB, tends to sort of just sit there on the other side letting the rest of play move on around him. So space opens up not through his movement but more everybody else's movement. The idea is to switch play to this guy and he then has the space to play a ball into the box or directly attack the goal.
With KDB out, the two Silvas have rotated in this role. So sometimes we attack down the left, sometimes the right. One Silva gets up and attacks and helps create overloads, the other sits back and serves as support. David staying to the left, Bernardo to the right.
However, with Mahrez playing, we tend to focus play down his side almost exclusively. The probable reason being that his ability to play the simple passing game we do is better than the other wingers's ability to. It was similar when Bernardo was playing out wide there last season (and against Huddersfield if you remember). But critically, both Bernardo and Mahrez like to play balls into the box (with Sané playing the Sterling role but on the left). It's maybe less traditional than what we normally do, but most of our forays into the penalty area, especially in the first half came down the right. One of those two would always look to get the ball in there. So David's focusing more on retaining team shape, recycling possession and occasionally getting himself into the box if need be. It was a similar role to one we've often seen Gundogan occupy; keep it simple and provide more use for us off the ball than on it, but important nonetheless.
He wasn't perfect in the role, and he did get a few things wrong, but he wasn't bad. Just played a more understated role, similar to how he did at the World Cup for Spain.