Coacher (1874)

“The umpire allowed too many players to act as “coachers.” The rules prohibit any but the captain and one assistant from acting as coachers. In this game, however, Anson made himself prominent, and kept too near the base-lines, as did others of the nine. This, umpires should stop.” New York Clipper June 20, 1874 {RH}

Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1879. “Michael Scanlon, of Washington, the ‘coacher’ and acting manager of the Nationals, is earning a fame in this respect greater than that of Harry Wright” (Chicago Tribune, July 6; Peter Morris). 

Earlier, non-sports, use:
“Mr. Burgess was an excellent coacher to aspiring students—many of whom, belonging to Cambridge, will hear of his death with regret.” The Irish People, January 30, 1864, p14. {Italics in original)
– “Coacher” is used again in an education sense in an Irish newspaper in 1870 (Ken Liss)