David Ortiz is entering his age-34 season and has a number of red flags on him to be concerned about for 2010. Other than being a DH-Only player, he had his worst OBP (.332) since 2002 (.339) and a career high in strikeouts (134). Ortiz had a much worse platoon split against lefties last year than his career line (.212/.298/.418 last year vs. LHP). Ortiz managed to hit over .250 in only two months last year, June and September, and only in June did he have an OPS over 1.000. While he did hit better in the first half than second half: .222 to .258 isn't enough of an improvement. Ortiz hit better with runners on but only hit .238/.339/.463 with Runners in Scoring Position last year, against a career line of .298/.403/.526. His fly balls hit went up 5% last year to just over 50% and his line drive rate has been cratering around 17%.
There might be a bigger picture problem here. If Ortiz struggles there are going to be more threats to his playing time. The Red Sox lineup will be flexible such that players like Adrian Beltre and Kevin Youkilis can DH a day and be subbed by players like Bill Hall and (for now) Mike Lowell. If Jeremy Hermida or JD Drew start out hot they will continue to draw playing time from Ortiz's DH spot.
I think the depth issue is something we like to worry about when we're looking at teams' outfielders, like who's going to be the 4th outfielder this year. For the Red Sox I think you have to worry if Ortiz gets off to a slow start because it will likely mean someone will be ready to take his job.