Hooker Leeming “tweaked” a knee in combat training and was on crutches after the session, but the injury is not as bad as first feared.
He will not be risked on Boxing Day, but could feature in the Rhinos next pre-season game at home to Bradford Bulls on Sunday, January 12.
Second-row Mellor injured his groin in the field session later in the day and will also miss out.
Young forward Muizz Mustapha has failed to recover enough from an ankle injury and has also dropped out from the team named last week.
Coach Richard Agar said: “We have had a really good pre-season in terms of injuries, but unfortunately just before our first run out we have picked up two on the same day.
“Both Kruise and Alex are desperate to play, having recently joined the club, but we have to be smart with both given the important part they will play in our season.”
Agar added: “Corey Johnson, who has been outstanding in training, will get the chance to impress at hooker and Alex Sutcliffe will now start in the second-row instead of off the bench.
“Muizz Mustapha rolled his ankle in training last week and we had hoped he would be right for Boxing Day but, like Kruise and Alex, we need to make sure he is fully fit so he can give the best account of himself.
“His spot on the bench will be taken by Wellington Albert. Jack Broadbent, who finished the 2019 season in great form for the Under 19s and Featherstone, will come on to the bench to fill the gap left by Alex Sutcliffe.”
Despite those setbacks, Agar insisted: “It is still a really exciting line up with Luke Gale and Rob Lui linking up for the first time and the boys have been in great spirits in training.”
The revised team is: Jack Walker, Harry Newman, Liam Sutcliffe, Konrad Hurrell, Ash Handley, Rob Lui, Luke Gale, Ava Seumanufagai, Corey Johnson, Mikolaj Oledzki, Alex Sutcliffe, Stevie Ward, Cameron Smith. Subs Brad Dwyer, Richie Myler, Adam Cuthbertson, James Donaldson, Jack Broadbent, Wellington Albert.