Amorosino returns after picking up an injury in the opening cup tie of the season at Leeds Carnegie, while Twelvetrees was injured in the Heineken Cup victory in Viadana back in October.
British & Irish Lions international Ellis reclaims his place at scrum-half.
In the forwards, England international prop Julian White makes a first start following his recovery from injury and he joins Boris Stankovich and Joe Duffey in the front row.
"It's great to have Harry, Billy and Lucas available again, and Julian White comes into the front row," says Tigere head coach Richard Cockerill.
"Sam Vesty, Aaron Mauger, Ben Woods and the rest of the guys will also benefit from the game-time out on the pitch. We're looking forward to it, as we do every time we welcome Bath at Welford Road.
"We're getting guys back from injury, we're getting options available and we're not too far short on the injury front now. Geordan Murphy is back in training, which is great news because he's a class player and he's club captain. He's probably 95 per cent of the way there now and he'll be back in action too shortly."
Tickets are still available from the Tigers ticket office, call the hotline on 0844 856 1880, book online or call in at Welford Road.
Leicester Tigers (v Bath Rugby, Welford Road, Friday January 29, 7.45pm)
15 Lucas Amorosino
14 Alesana Tuilagi
13 Billy Twelvetrees
12 Aaron Mauger (c)
11 Matt Smith
10 Sam Vesty
9 Harry Ellis
1 Boris Stankovich
2 Joe Duffey
3 Julian White
4 Calum Green
5 Ben Kay
6 Ben Pienaar
7 Ben Woods
8 Brett Deacon
Replacements
16 Tom Youngs
17 Robbie Harris
18 Ryan Bower
19 Dan Hemingway
20 Tom Armes
21 James Grindal
22 Andy Forsyth
23 Johne Murphy
Bath Rugby
15 Jack Cuthbert
14 Joe Maddock (c)
13 Tom Cheeseman
12 Ryan Davis
11 Rhys Crane
10 Nicky Little
9 Scott Bemand
1 Nathan Catt
2 Pieter Dixon
3 Duncan Bell
4 Peter Short
5 Scott Hobson
6 Daniel Browne
7 Josh Ovens
8 Ben Skirving
Replacements
16 Mark Lilley
17 David Barnes
18 Aaron Jarvis
19 Andy Beattie
20 Guy Mercer
21 Jonny Faamatuainu
22 Matt Keyte
23 Matt Spelman
