TO be honest it could be anywhere inbetween both extreme's, he could get battered easily, or beat a out of shape Barrera whose not going to be near his best.
It could be seen by either side as a final chance at a pay day. Barrera, biggest fight he could get for a little extra retirement money and is so desparate that he'll fight anyone. Likewise, he could be the only "big name" who would remotely bother fighting Khan, frankly any real current great boxer will see Khan as nothing but a small time fighter, at least for the moment. So if they think Khan will never win anything significant and never match to a champion, this could be the biggest payday they have left also.
But the problem is, Khan's all show, no skill, no talent, speed but no power, very fast small punches in crap combo's, but lacking any real technique or ability to penetrate a real defence, can't take a punch either. A previously top boxer shouldn't really have any trouble pounding a poor boxer tbh. De La Hoya was older, in worse shape and was fighting a true champion.
Its hard to see Barrera being anywhere near his best, but even so, that doesn't mean he can't take Khan. Interesting match up though.