Jim Benton has the most receiving yards in a game in his first 9 games, with 303 yards versus the Lions on November 22, 1945.

most receiving yards in a first 9 games
Interpreted as:
most receiving yards by a player in a first 9 games
11/22/1945Benton10/30/1960Carlton10/9/1960Maynard11/27/1938Tinsley9/24/1937Tinsley11/6/1960Dubenion10/23/1960Maynard11/6/1960Groman12/5/1937Tinsley9/24/1960Doran10/21/1945Benton10/30/1938Hutson10/10/1937Tinsley11/6/1960Taylor10/16/1960Groman11/7/1937Hutson11/14/1937Carter10/9/1960Doran10/2/1960Powell10/15/1950Lavelli11/2/1941Dewell11/19/1939Ramsey9/9/1960Carmichael303177174167167161156155155154151148148140140140139134134131131131130
Results may be incomplete. StatMuse has complete game-level data for receiving yards going back to the 1960 season.