Ewens is suffering from pain in his shoulder but is expected to be back in contention for the return leg.
Elliott made his last appearance for the Titans back in December in the 10-17 defeat against Narbonne in the Parker Pen Challenge Cup.
He has been out of contention since then with a bout of glandular fever but blood tests have shown that there is now no trace of the illness in his system.
Aside from the one enforced change, Steph Nel has named the same starting 15 that overcame Gran Rugby of Parma at Millmoor last Sunday.
Viadana: C Steyn; T Beim, A Masi, H Crane, M Dolcetto; B Feeney, M Mazzantini; M Savi (capt), V Jimenez, E Brancalion, C Bezzi, T Bowman, M Phillips, L Persico, G Hayter
Replacements: E De Bellis, P Gazzola, F Gumiero, F Angeloni, S Bortolussi, A Accorsi, S Ceppolino
Rotherham Titans: B Stortoni; A Elliott, J Pritchard, P Jones, M Wood; J Benson, J Rauluni (capt); N Lloyd, C Johnson, C Noon, G Kenworthy, L Gross, L Greeff, C Short, G Lewis
Replacements: S Bunting, J Rawson, J Pendlebury, M Rhodes, M Umaga, J Strange, C Harrison
Referee: R McDowell (Ireland)