Yoshida - definitely. I'm sure he was thought of as one of the starters at the beginning of the season, and would be if Fonte wasn't playing so well. Hoolveld was actually played ahead of Fonte last year so they must consider him a decent player as well; probably the least technically gifted of the 4 but still useful and able to score the odd goal as well (providing its a Tuesday!). I think this is basically a position of strength for us in terms of who starts and who we have in reserve. No doubt you can always find better, but maybe the money could be more usefully spent elsewhere on the field. I've just realised you were talking Champions League So my actual answer then is, I've no idea!
Hooiveld was picked ahead due to injury to Fonte, and then he kept him out for a while. Hooiveld's a perfectly good CB, but he lacks mobility, compared to the others, and has an unfortunate tendency to stick a leg out, which succeeds in merely diverting a shot past his own keeper, instead of blocking it entirely. But he's dominant in the air, and can score goals on Tuesdays, [Bugger, it's a Wednesday, I'll change that] or [cough] other midweek match days.
Hooiveld and Yoshida are not good enough for CL as a pairing. But Saints are in all likelihood not going to need to worry about that next year. I believe however, that Saints could just about tread water for a few games if we had to play them both in PL. And if it is only one of Fonte/Lovren that gets hurt, we would probably not miss a beat. Or maybe just a slight half-beat. But our starting attacking mids are not only not good enough to play in CL, they probably aren't good enough even to get us to CL. So not seeing the priority on a DC unless there is someone out there who knocks us out. I couldn't resist looking up Janssen on FM. He's pretty much a Fonte clone. Slightly faster, but not by much. The two are more-or-less interchangeable. Janssen is slightly superior as a pure DC (and thus rated 1/2 star higher by Tord Grip), but Fonte is slightly more well-rounded overall. Lovren is much better than both, although in the game he is hurt badly by a poor Composure score, probably because of last season. If his composure isn't actually that bad then he is miles ahead of Janssen. As a side note, Yoshida and Fonte are pretty close, Yoshida might even be a smidge better. Hooiveld is a half star worse than Fonte so slightly worse but not dramatically.