This will probably strike a nerve with several Cubs fans since Mark Derosa was a fan favorite. So please take it into consideration that I am merely making a comparison based off of computer stats, not personal opinion. Let's step back and compare DeRo with a possible internal replacement, Mike Fontenot.
Mark DeRosa was a slightly below average defender at second base in 2008 (UZR -8.8), as well as slightly above average offensively at the position. his replacement should be......
Mike Fontenot. Fonty's UZR150 (16.5) is way better than Derosa and Fangraphs predicts that he would be a significantly better defensive second baseman over the course of a season than Derosa. However, 16.5 is a bit of a stretch since he was not an every day player. They suggest that a modest adjustment would be to take half of that number.
Now lets move to offense for a second. Mark Derosa is coming off of a career year and he will more than likely never duplicate that production. Mike Fontenot had an excellent offensive season as well but mainly faced right handed pitchers. Derosa's career average would be around .280 13HR and 65RBI. That is probably a more fitting projection than the 21 and 87 he had this season. Mike Fontenot's career consists of roughly 500 at bats (essentially a full season). A number of those at bats were pinch hits as well (which add difficulty to those at bats). Over his career Fontenot is a .290 12HR and 69RBI. those are very comparable to Derosa's career average. So, from that we can conclude that Fontenot is a comparable offensive player.
Mark Derosa is projected with a WAR 2.0. this means that as an overall player Mark Derosa adds 2 wins over an average replacement.
Mike Fontenot is projected with a WAR of 3.4 over a full season. That would mean that Fontenot is 3.4 games better than the average replacement and 1.4 games better than Derosa. So, from a statistical point you could make a strong argument that Mike Fontenot is a better player and that by losing Derosa and replacing him with Fontenot the Cubs actually upgraded at second base internally.