Lets put this into perspective. What happened last night was not an MMA fight, it was a kickboxing match with 4 oz gloves. It was no different then the Slice/Petruzelli, and why people are glancing over that fact is beyond me, this wasn't a boxer beating an MMA guy at MMA. Tim Sylvia, one of the most misguided MMA participants ever,  agreed to stand with a far superior striker, and got KO'd. Shocking? Not at all. With 4 oz. gloves anyone can get knocked out, does that mean one fighter is a better fighter than the other, no. Serra caught GSP, Randleman caught Cro Cop, the list goes on and on.   Along with being misguided Sylvia has to be the most Tailor made name heavyweight, for a boxer to fight. He has no wrestling, Submission, or kicking ability, and has made a career of keeping fighters at a distance with his long arms. Several unknowns have knocked out name fighters in their first fights on a big stage, some examples:

Houston Alexander Unknown entering UFC.

Ko'd Top contender Jardine in 48 seconds

Ko'd UFC vet Sakara in 1:01

0-3 since, stopped in first round in all 3 bouts, with an average of surviving 2 minutes and 45 second a fight 

Sokoudjou 2-1 before entering Pride

Ko'd Top 10 fighter Nogueria in 23 seconds

Ko'd top 10 fighter Arona in 1:59 seconds

2-3 since.

Paulo Thiago Unknown BJJ guy

KO'd Top 10 Josh Koschek in 1st UFC fight.

Future Unknown

Dos Santos Unknown before entering UFC

KO'd Top UFC contender Werdum in first round of first fight

KO'd Stefan Struve in 2nd fight

Future Unknown. 

If fighters like Alexander (a street brawler) and Sokoudjou (a Judo guy) can knock established MMA talent in their early fights, why wouldn't a pro boxer be able to in a fight which was said to be a stand-up fight? Does than mean Street brawling and Judo is better than MMA? 

Marcus Davis was a 17-1-2 pro boxer, but was a mediocre MMA fighter until he expanded his skill set, now has 50 percent of his wins via Submission. Alesio Sakara a 7-1 Pro boxer, has yet to round out his skill set and has been KO'd in 3 of last 6 fights to "inferior MMA strikers".  

And lastly as for Mercer, he is 0-2 in K-1 (according to boxing fans K-1 fighters are left overs that couldn't compete in "real" boxing) and 1-1 in MMA with a SUBMISSION loss to Kimbo of all people. This boxer has gone outside his comfort zone and has won one out of four. Impressive?