Key has not scored a first-team century since May 5 last year, and after a difficult start to the season, has opted to spend his 36th birthday on Tuesday with the 2nd XI rather than facing Glamorgan at Canterbury.
He has averaged just 14 from six innings to date with a top score of 34 against Lancashire at Old Trafford. He made eight and seven against Leicestershire in the drawn match at Canterbury last week.
Key decided not to play against Glamorgan on Sunday’s opening day, in which Kent scored 330-8, with Daniel Bell-Drummond making 123 and Darren Stevens 50.
Sam Northeast led the team in Key’s absence and hailed his skipper’s “courageous” decision.
“Rob made the decision himself and passed on his views of the senior management,” Northeast told the Canterbury Times.
“He’s not felt in the best of form and thought it best to drop down to the seconds to get some confidence and runs under his belt.
“It’s a courageous thing for any captain to do and I have to respect him for that decision, absolutely.
“He’s done it for the good of the team, which is creditable in its own right. I’m sure we’ll see him back in a Kent shirt very soon.”