Prop Castrogiovanni returned from World Cup duty with Italy to take a place on the Tigers bench last week and made his seasonal debut as a half-time replacement, while Davies has been in the matchday 22 for each of the opening four games and is now given a starting opportunity.
Acting head coach Richard Cockerill has also selected Ben Youngs for his first start alongside Paul Burke at half-back, while Boris Stankovich returns after injury. Sam Vesty is named at centre after also missing the Gloucester game, and Ayoola Erinle comes in on the wing for his first start since the opening-day win at Bristol.
Cockerill said: "Some of the guys have played four full games on the trot while others have had to wait for their opportunities. The guys have stepped up really well during this opening period and now we are looking for them to carry that on and for others to take the opportunities as they come along.
"We have lost one in four so we have not done badly so far, although we were disappointed with the result against Gloucester last week and we have to learn from that defeat.
"Saracens have also had a good start, they are a good side, well coached and with some good experience in the squad, but we are looking forward to the challenge and I'm looking to the guys for a big performance."
Leicester Tigers (vs Saracens away, Sunday 3pm)
15 Geordan Murphy
14 Johne Murphy
13 Ollie Smith
12 Sam Vesty
11 Ayoola Erinle
10 Paul Burke
9 Ben Youngs
1 Boris Stankovich
2 Mefin Davies
3 Martin Castrogiovanni
4 Louis Deacon (c)
5 Marco Wentzel
6 Brett Deacon
7 Luke Abraham
8 Jordan Crane
Replacements
16 Benjamin Kayser
17 Julian White
18 Richard Blaze
19 Tom Croft
20 Christophe Laussucq
21 Matt Cornwell
22 Tom Varndell
