Much of the discussion that I've seen on Boston's recent signing has centered on the fact that Grady Sizemore has missed two full years, and a lot of time due to injury in the two years prior to that. There is skepticism that he can come back at age 31 and be an effective Major League hitter.
Well, there is precedent. Here's a good center fielder returning to MLB at age 31 after missing more than two full seasons:
1942 - .305/.376/.498/.875, OPS+ 147Obviously, very different circumstances, but...
1943-1945 - Did not play
1946 - .290/.367/.511/.878, OPS+ 142
Washington's Stan Spence also played CF at age 31. He had only missed one full season.
1944 - .316/.391/.486/.877, OPS+ 155Luke Appling missed his age 37 season and hit .368/.478/.526/1.005, OPS+ 195 as a 38 year-old. Tommy Henrich hit .251/.358/.411/.769, OPS+ 113 as a 33 year-old after missing three full seasons.
1945 - Did not play
1946 - .292/.365/.497/.861, OPS+ 145
If Sizemore cannot physically stay healthy enough to play MLB, then he'll cost the Red Sox $750,000 and a roster spot and some time spent determining that. If he can, then age and time missed aren't adequate reasons to think that he couldn't have another one or two good seasons.
So it's a good signing. The likeliest scenario is that he gives them almost nothing and costs them almost nothing. There's a small potential for a positive contribution that outweighs any of the likely costs. But he can't hurt them unless he's a) healthy b) bad and c) in the lineup every day. Which would be the manager's fault, and still wouldn't make it a bad signing.