Well, David Wells was not really unknown, but he was definitely near the end of his career by the time he joined the Red Sox. Wells had been successful for several years with the Blue Jays, Yankees, White Sox, and several other teams and he joined the Red Sox in 2005 at the age of 42.
Nevertheless, Wells had a pretty decent season in '05. He won 15 games and pitched to a 4.45 ERA with 107 strikeouts. Wells was one of the better pitchers for the Red Sox that year, as many of their other starters had injury problems or were ineffective. Wells's weight was always an issue, but that was part of what made him such an interesting pitcher to watch.
2006 found Wells dealing with a variety of injuries and ineffectiveness. He did start the only game I have seen at Fenway Park to date, and he took the victory in that game, despite leaving early after a line drive caught him on the knee.
Later in the year, Wells was traded to the Padres for future backup catcher George Kottaras. He did pitch one more year before retiring at 44.