When famous celebrities appear on The Office, they usually play a role that is completely unrelated to Dunder-Mifflin. This was true of Christian Slater, Conan O'Brien, Jack Black, Jessica Alba, and Cloris Leachman. The reason this is done, is to maintain the documentary feel of the show, as having famous celebrities playing in-universe characters ruins the suspension of disbelief. As Steve Carell is leaving The Office, Will Ferrell, was mysteriously cast as Michael's temporary replacement. It is even more disheartening to hear Paul Lieberstein discuss (link here) well known celebrities appearing on the show, and possibly taking over Michael's position. When a pillar in the show's formula has been betrayed, the quality of the show is inevitably lowered.
Training Day is the series' worst episode since Manager and Salesman. Will Ferrell's character failed to make a dramatic entrance, and any relevant effects. Deangelo is a bit of an idiot, and the employees sucked up to him in really dumb ways. Andy is stuck doing clumsy slapstick, Darryl got his sister to drop off boots for him, Jim and Pam showed off their baby, Kevin put on his toupee, etc. The only exception was Dwight, who in his eternal quest for a promotion, also inadvertently discovered that Michael did not recommend him for a management position.
Michael, instead of acting like a man who just got happily engaged last episode, was reduced to attempting to preserve his influence in the company. In a game of one-upmanship that he was only playing with himself, he resorted to micromanaging Erin's phone greeting, getting shaved in the office, and triggering Deangelo's peanut allergy. This regression is not only unfitting, but it is also not at all funny to watch.
This episode made attempts at humour, but did not succeed. Training Day was dull, and signaled that the show is close to jumping the shark. If it has not already jumped the shark, it will soon, as the remaining episodes of this season will leave the viewers on the horrible cliffhanger of wondering who is going to replace Michael Scott. The question should not be who is taking Steve Carell's place, but how bad will the show be without him? As the center point of The Office has always relied on Michael's poor management and how other people reacted to him, a new manager will completely defeat the point of the show. Of course, unless the replacement's name is David Brent.