The Cubs optioned Ferguson to Triple-A Iowa on Thursday, Jared Wyllys of AllCHGO.com reports.
Ferguson had pitched each of the last two days -- including throwing 29 pitches over two scoreless frames versus the Dodgers on Wednesday -- so he's being replaced by a fresh reliever. The 32-year-old has logged a 3.45 ERA, 0.77 WHIP and 19:2 K:BB over 15.2 innings for the Cubs, so he's likely to be back with the big club sooner rather than later.