Casey McGehee was a bit of a mystery. He was claimed, we weren't sure we got it, and then he had an awesome spring training, putting up a .370/.370/.722 line so far. We should not really change our evaluation of him as a player based on a good spring training performance, but even if Lamb and McGehee had performed similarly in spring training, I might still choose McGehee.
McGehee is not going to be a great hitter for this team. His minor league track record is pretty average-looking (for an upper-level minor leaguer, not for a major-leaguer). His wOBA came in at slightly above his AA league's average in 2007, and his solid 2008 was a slightly below average performance. Because he's hit like an average minor leaguer the past two years, it would be irresponsible to project him for much in the majors. He's not going to hit for a high average, and the trend is about a .050 gap between his average and obp-- so we are probably looking at something like a .260/.310 true-talent major league hitter.
McGehee does bring a few nice positives to the table, though. Defensively, he's versatile and appears to be an average defender at third. A majority of his minor league playing time has come there, and he's rated out as a +10, +4, +1, and average defender in his four full seasons above A ball. Infield talent increases with the level of play, so it would be no surprise to see McGehee rate out as an average defender or just a bit below average at third base. The general scouting consensus seems to be that he is a quality defender there, but nothing special. McGehee played very little at second base in the minors, so there is little data to go on at that position-- although most indications are that he is capable of starting there. He has not played at shortstop since 2006, so I doubt there are any plans of putting him there even if Counsell is unable to play at some point.
McGehee also has some nice slugging ability. He has been a 10-homer per year type guy in the minors, so I would expect him to hit several in his duties this year-- somewhere in between our two fill-in third basemen of 2008, Counsell and Branyan. He forms a nice infield backup duo with Counsell-- good slugging and low obp righty and low slugging and good obp lefty.
Finally, McGehee should be compared against Lamb. It is easy to forget just how bad Lamb was with the Twins, he managed only a .276 obp and is a poor defender at this stage in his career. He also has never had a large platoon split. Now we do not have to worry about a potential third base platoon, I would be more worried about this situation if Ned Yost were still the Brewers manager, but the presence of two lefty third basemen on the Brewers roster might have been a temptation to platoon Bill Hall if he struggled to hit righties early in the year. Given Lamb's poor projections v. right handed pitching and his bad defense, playing Hall every day is probably more desirable than a platoon with Lamb-- and Hall on his own has much more upside. McGehee and Counsell will surely get starts at third, but Lamb provided the biggest threat to Hall's starting status, and now he is gone.
McGehee also becomes one of the two righty pinch hitters for Ken Macha, meaning he will see plenty of action against lefties late in games.
He may not be the best utility guy you could ask for, and Cub fans will probably laugh upon hearing he made the roster, but this is not a bad move. In the grand scheme of baseball, picking your co-backup at third base is not a very important decision to make so long as you can avoid disaster. I do not think you can go wrong by picking the guy with the solid glove, even if he's not a great hitter. And maybe some luck will carry his "hot streak" into the start of the season.
I still think they should have signed Branyan.