Lesnar was so inconsistent in his short MMA career that it's really hard to know how the Brock/Fedor fight would've gone. If you break down Lesnar's performance/opponents/etc, you can see why:
 Min-Soo Kim - Squash match; doesn't tell us much
 Frank Mir - Brock went at Mir very recklessly and paid for it. However, some folks say that in his 2nd fight, he showed promise if he could control that recklessness.
 Heath Herring - Here, Brock showed intelligence in his game planning. He was adamant to not get over-excited and rode Herring out.
 Randy Couture - Although Couture was a formidable opponent, he was still nearly 65 lbs at a disadvantage come fight time and was coming off a 15-month layoff. Couture seemed to control Lesnar for most of the fight [surprisingly] in the clinch and it seemed that he was going to ride his way into a decision victory. However, he got caught behind the year with a punch and the rest is history. Although Brock won the fight, his performance generally was poor.
 Frank Mir - This was Brock's best performance in his whole career. He intelligently controlled Mir, beat him and then TKO'ed him.
 Shane Carwin - First round was brutal for Lesnar; Carwin then gassed out badly and then got choked out. In terms of performance, it wasn't good either but he definitely showed a lot of heart in this fight.
 Cain Velasquez - He pretty much got beat up. Very bad performance.
 Alistair Overeem - Looked even worse than the Cain fight. Very bad performance once again.
Fights 6,7,8 where he had very poor performances was all post-diverticulitis, so people may [or may not] excuse that one. Fights 1-4 didn't tell us much about his potential even if he won and became the UFC champ. Fight 5 (Mir II) was a great one in performance and everything but it's hard to judge one fight on his own on how he would've done.
I honestly have no clue how Lesnar would've looked against Fedor even if it was immediately after Mir II. Would he have caved after one two bombs from Fedor? Would he have controlled Fedor like the Mir fight at least for a round or two? Would he have been armbarred?
We can all make speculations but if we were going with history, performance and consistency, then it's likely that Fedor would've beat Brock and pretty convincingly. He may have done well for a round in terms of takedown and some control but I can very likely see Fedor with his arm raised at the end in most scenarios.