The former London Irish flanker, who played 40 minutes in the 33-28 victory against Rotherham Titans last Saturday, is being looked at as a possible replacement for the injured Sam Dickinson.
Dickinson has been forced to have surgery on a neck injury and will miss the first three months of the season, leaving Saints in the market for a replacement.
And Fisher could be that man should he impress on Saturday, when he will be on the bench for the game against Leinster (2.30pm) before starting the evening match against Moseley (5pm).
Saints could use as many as 46 players over the course of the two matches, with Tom Stephenson, who has been named in both starting line-ups, guaranteed to feature twice.
International stars including Ben Foden, Alex Corbisiero and Tom Wood get their first action of pre-season from the off against Leinster, with Dylan Hartley and George North among the replacements.
The team to face Moseley has a more inexperienced look to it, with boss Jim Mallinder opting to field a similarly experimental team to the one that came back to beat the Titans last weekend.
Saints team v Leinster: Foden; Elliott, Waldouck, Stephenson, K Pisi; Hooley, Fotuali’i; Corbisiero, McMillan, Ma’afu; Craig, Day; Dowson, Wood (c), Manoa.
Replacements: Hartley, A Waller, Denman, Lawes, Clark, Fisher, Dolan, L Dickson, Myler, Burrell, G Pisi, North, Wilson.
Saints team v Moseley: Wilson; Skelcey, Hutchinson, Stephenson, Packman; Olver, A Day; E Waller, Marshall, Denman; Onojaife, Woolford (c), Fisher, Harrison, Dolan.
Replacements: McMillan, Hobbs-Awoyemi, Parkins, Foster, Conner, Kendall, Taylor-Dawes, Robinson, Hooley, Marlow.