From what I saw before he went down with injury, Mueller winning the batting title last year seems like it was a fluke. Sure, he's a decent player, and he plays o.k 3b, but Youkilis might be on par with him right now, with the potential to surpass him at any second.
Here are Mueller's stats from this year so far offensively:
2004 BOS 36 145 23 38 7 0 5 21 15 21 0 1 .262 .333 .414 .747
Not too great. Youkilis:
Career 29 103 21 28 7 0 3 14 15 22 0 1 .272 .372 .427 .799 about the same, but in about 40 fewer at bats. If a guy who probably should be in AAA right now is outhitting your day to day 3b, something is probably not going well with your day to day 3b.
If Mueller comes back, and Tito desperately wants him to play, I guess you do have to send Youk back down, so he can play everyday as JM mentioned and continue to hone his skills. It's better than riding the bench, and he'd understand that its for the best.
It'll be interesting to see how things play out I guess.