The 23-year-old was tackled from behind by Barnet's Calvin Gray five minutes into the reserve victory last week.
Tests have shown that Rowlands' ankle was worse than expected and he faces around a month on the sidelines.
Meanwhile, defender Leo Roget has agreed non-contract terms with The Bees, in order to ensure his availability to manager Wally Downes' squad.
The 25-year-old is still in negotiations over a proper deal at Griffin Park and with his registration expiring at the end of last month, Brentford have signed Roget again on a non-payment deal for the moment.
In other news, fellow defender Jay Lovett has joined Conference side Hereford United on a month's loan.
The 24-year-old has fallen down the pecking order at Griffin Park and spent a loan spell at Division Three side Lincoln City last November.
