If you experience noticeable DOMs
Delayed onset muscle soreness - Wikipedia, the free encyclopedia next day after riding then taking extra protein may help with that.
How much will it help (or not) depends on too many factors.
When I was doing weight lifting someone recommended to take BCAA before/during workouts and it did made noticeable difference, recovery was faster and less DOMs.
I was very sceptical initially but since I did exactly same routine for years I could pretty much tell how I would feel next day normally vs days I took BCAA so it was hard to argue.
To my limited knowledge most of sports "recovery" drinks have some amount of BCAA.
Unlike other forms of protein (especially "slow" ones like casein) BCAA can be absorbed very quickly and used by your body to repair & rebuild damaged muscles almost instantly (5-10mins after consumption - check with wiki here as I forgot details).
Essentially this allows your body to repair itself faster, no other miracles here.
You probably do not want to take any casein while you are riding - it will take hours for your body to absorb it and there's zero benefits from consuming it while you on the bicycle.
Casein shakes usually consumed right before going to bed - the logic here is that you keep that extra protein supply flowing overnight.
All above only makes sense if you actually have DOMs / stressing your muscles beyond what they normally can handle and cause some damage (hence DOMs next day).
If you do not have that then taking BCAA or any protein really is just extra consumed calories.
Also keep in mind that extra protein alone will not help recover faster, you will need some carbs (for energy supply) and of course rest.
Ideally (and especially if you genetically gifted) each damage -> recovery cycle should result in better/bigger muscles (
http://en.wikipedia.org/wiki/Supercompensation ) but for most it takes long time to achieve and hard to notice.
During my hard rides I still use BCAA as described above.
I'm using this brand
Amazon.com: Optimum Nutrition Instantized BCAA 5000mg Powder, Unflavored, 345g: Health & Personal Care for 3+ year now.
Simply adding 3 teaspoons to Gatoraid bottle, shake & pour to bicycle bottle and drink while riding.
Drinking that mix has same effect on me - I usually can ride pretty hard day after and two days after I'm almost at 90-95%.
Just keep in mind that positive effect of all those protein supplements rather minor, you can simply eat better to get same results.