Sakaguchi, who usually races in the GT300 class for the K-Tunes Racing Lexus outfit, will make his GT500 debut in SARD's #39 GR Supra alongside team regular Yuichi Nakayama.
He takes the place of reigning SUPER GT champion Kenta Yamashita, who was drafted in for last month's pre-season test at Fuji and the opening round of the series, with ex-Formula 1 racer Kovalainen still unable to make the trip from his native Finland to Japan.
#96 K-tunes Racing RC F GT3
Following Sakaguchi's call-up to SARD, Nitta will be joined in the K-Tunes Lexus by another experienced driver, Shigekazu Wakisaka.
The younger brother of three-time GT500 champion Juichi - incidentally now team director at SARD - was replaced in the Saitama Toyopet Green Brave line-up this year by Kohta Kawaai, who delivered the team its first win in the Fuji opener alongside Hiroki Yoshida.
Elsewhere on the entry list, the #35 Team Thailand Lexus that was missing from the Fuji opener is listed, with Sean Walkinshaw and Nattapong Horthongkum as drivers. If the team makes the trip, it will boost the entry to 30 cars in GT300 and 45 in total.
A couple of other GT300 drivers that missed the opener, Christopher Mies (Hitotsuyama Audi) and Alex Au (X Works Audi), are also tentatively listed.