From Hardball Talk, we learn the details of Welington Castillo's knee surgery:
Mark Gonzales of the Chicago Tribune reports that Cubs catcher Welington Castillo underwent arthroscopic surgery — as scheduled — this weekend on his banged-up right knee. The procedure was deemed a success and Castillo is expected to be fully recovered in 4-6 weeks.
The Hardball Talk link quotes Gonzales, but I can't find anything on the Tribune website, nor on Gonzales' Twitter feed. In any case, this surgery was expected after the knee injury suffered by Castillo September 20 against the Brewers.
Castillo started slow in 2013, but hit .288/.388/.475 with six home runs and 17 RBI in 41 games after the All-Star break. This past season was his first full big-league season and the Cubs are hoping he'll take a big step forward and have a bigger year in 2014. As noted above, Castillo is expected to be ready for spring training.