Yeah, he's not bad defensively or anything, for a creative midfielder he's pretty much as good as you can expect and is an excellent tackler. Just in big games, we need someone more like Carrick to hold his position and keep the defensive shape and organization, but herrera's pressing and eye for a through pass is also very important so he'd still be in. Our defence is very fragile of course, so we need the extra man in midfield like Carrick for when we don't expect to dominate possession. If we had Rio and Vidic at their prime, then we'd be okay with Herrera-Carrick or herrera Blind I think
The bolded statement is a big reason Chelsea suffers with Fabregas and Matic in the middle. We cannot make the same mistake as they do. Blind and Herrera are quite aggressive off the ball, and both will get themselves dragged out with their aggressive style. For this reason, we can't go with Blind and Herrera unless Blind plays a much more disciplined defensive game (which he definitely can).
Still, though, I am happy to see Herrera prove that he isn't some defensive liability from midfield. When playing in central midfield, Herrera showed a great amount of discipline and provided good defensive support for Blind. He wasn't gung ho in his defensive approach, and he did a good job in recovering the ball as well as slowing down Sunderland. He does show some defensive awareness and a decent reading of the game ahead of him, but I don't think it's enough for him to play in a midfield 2 base against top teams. However, he can improve on this like Modric, Pjanic, Fernandinho, Rakitic, and Gundogan did. If he cannot do this, then he doesn't have the physical qualities to make up for that, which players like Nainggolan, Ramires, Fernando, etc., have.