^kind of sudden right? there wasn't really any foreshadowing that I noticed, the artist probably could have done it better. but maybe thats the exact shock that he intended
There were definetly some hints. Besides a similar appearance, there was the part with the attack of Annie in titan form, which was explained in the same chapter of the revelation [Why she ran to the right wing and why she stared at her hand after taking a hit by Reiner].
Then the part with Conny's village, where Reiner obviously was avoiding the topic of the talking giant in Conny's house and was reacting rather harsh towards Conny afterwards, in contrast to being so suspiciously nice the whole time. Oh yeah, the part with the herring can.. It seemed that he knew too, what was written on the can.
Most of this is very subtle and I guess that was intentionally so we get quite a shock, just like you said.