No, the DBs may not be good enough for man, but zone aint workin, so it is worth a shot. With patrick back, the D may be able to play zone. Also, I think there are Riders CAN play man, and have the personnel to do it well, they just are not starters. Between Frazier, Patrick, Butler, McKenzie, and Eddie Russ I believe Man coverage is doable.
Also, when one has no confidence in the ability of their personnel to play man, or whatever the reason is behind not using it, that leaves you with the need to play zone. Zone coverage really only works if you can get heat on the QB in about 3 seconds...not even hit, just make him throw sooner than desired. The problem is that the Rider line can not get that pressure with 4 men, even 5 really. Therefore, you put 3 guys on the line and add someone to the secondary. The idea being that you either split the middle shallow zone into a 3 through 7 yard and a 7-15 yard zone, or into a left center, right center zone. The Riders have been using primarily the front back split, which protects well against the 12 yard pass. It does not however cover the 6-7 yard hot route nearly as well, and you get passed on underneath with high percentage throws. Basically, what they are doing by putting 3 guys on the line is trying to have tighter zones. Zone coverage is a very difficult thing to get good at, generally taking 5 or so games unless the secondary has historically played together in zone defense, as you must be able to anticipate what each other are going to do (unlike man...simple...that is your guy...cover him...the safety will roll back for zone over). Think of it kind of like a like a line in hockey...the longer they play together, the better they play together, except you have 8 guys, not 5. When it works, it works well, but it is kind-of a thing of the past as QBs study footage enough to know how to tear up most Zone defenses...really fast releases (3-5 yard timing routes such as outs, slants, posts, etc) are very hard to cover in zone. Really, you are going for a quick tackle in these situations, not breaking up the play (the fact our tackling is about as bad as last season does not help). When plays drag on, and receivers get deep, zones get stretched, and targets get left wide open...that is not a breakdown in defense, it is simply the flaw of zone coverage.
After 3 seconds a receiver is crossing zones. take the left zone as an example...the WR runs a fly, the DB in the shallow left zone is now at the back of his zone. so, if a receiver runs a slow out to that flat he is wide open, unless the LB (or perhaps an inside DB) has the recognition of what is happening, and drops his zone to roll right. At the same time the next LB was to know to roll right/center to cover the first rolling DB's area. To get your secondary on the same page and thinking alike takes time, and must be frustrating because they are continually getting burned due to lack of pressure. In all fairness, the zone D has looked better in each game, and did look really good for the first 1/3rd of the Hamilton game until they got burned out from playing too many minutes. Zones are a very complex strategy. Perhaps the biggest problem is that when someone goes down, even if for only 3 plays, somebody else has to come in and try to fit into the mold...obviously harder than man coverage.
Personally, I am a fan of zone over man defense. If you are unfamiliar with this term it is when you play man coverage with a designated person or 2 dropping into a zone (often mis-viewed as double coverage). That zone coverage is often the safety. Man over zone is the opposite...it is a zone coverage with one person getting an extra man on him in man-to-man coverage...generally one of the speedsters who runs a lot of slants/crosses, as he then has coverage while crossing zones.
At any rate, what I am trying to explain is why I think it is too early to write off the defense (zone takes gelling time especially when new to the team), though I still don't think it will work without a good pass rusher. I still prefer man coverage, mainly because it is less prone to breakdowns. When a man coverage gets blown it is typically a monster play, though even playing zone we have been burned deep a few times, and the point of zone is to NOT give up the big play...just quick hitters, fast tackles, and force an error or hold them short of a first down within a couple tries on a drive. Also, we have very good linebackers, and the added advantage of zone is that it generally stops the run better (because you are watching the ball, not the player you are covering), and having a solid LB core is generally the first step in good man to man coverage (so they can stop the run).
sorry for the lengthy post, congrats if you read it all, and hopefully there is a person or 2 that will find some of the briefed down explanations a bit helpful, though many are likely fairly familiar with all of this already.