The 21-year-old saw summer signing Saidy Janko start in defence in place of the injured Mikael Lustig when Celtic made nine changes against Dundee United on Saturday, while Efe Ambrose is another option for the full-back spot.
The Reading-born defender will have to wait for his debut, though, as Saints take on Celtic at Parkhead on Saturday.
Fisher will also face competition at McDiarmid Park with Dave Mackay and Joe Shaughnessy both favouring the position.
Fisher has made 21 appearances for Celtic, including 11 starts, making his debut in October 2013.
A Celtic statement read: "Everyone at Celtic wishes Darnell all the best for the season ahead with St Johnstone."
Source : PA
Source: PA