John Mueller from Google stated on Twitter that one bad link cannot make Google take manual actions and make search rankings go down. A site needs to have many more issues than simply a bad link.
When you site does all the good things, just one bad thing is not going to get you a penalty like manual action. Google will not dole out a severe punishment because you have one bad link. The action will only be taken when about fifteen percent of the links do not follow the guidelines set by Google.
Someone had mentioned on Twitter that Google has a nofollowed policy and thus, they take manual action. John replied to him saying that it can be that the link in question might have been nofollowed but that does not mean a single link can bring on the manual action.
See the forum discussion at Twitter.