The set-piece specialist will also make his Heineken Cup debut after successfully returning from a shoulder injury in Gloucester's 33-24 victory over Worcester in the Guinness Premiership last Friday.
Nieto will line-up in an all-international front row alongside Welshman Mefin Davies and loose-head Christian Califano, who has 33 European Cup games to his name after spells with Toulouse and Agen and replaces Nick Wood.
"The fact that Nick Wood is not starting is not a selection issue because all the three props will play a significant part in the game," head coach Dean Ryan told www.gloucesterrugby.co.uk
"I have been delighted with Nick's form - he has been fantastic for us this season - but a little bit of extra experience will be crucial."
Ryan is also able to welcome back Italian skipper Marco Bortolami in the second row and he will partner Adam Eustace, who has made 21 competition appearances.
The back row trio of Peter Buxton, breakaway Andy Hazell and number eight James Forrester, who was named September's Guinness Premiership player of the month, is unchanged.
There is also continuity to the backline and it remains unchanged from the one that triumphed at Sixways.
The only alteration comes on the bench, where Rudi Keil is preferred to Mike Tindall, who has yet to fully shake off a calf injury. Explaining the decision, Ryan added: "Last Friday was an indicator of where we felt Mike was with his calf injury and at the moment we feel we don't want to risk him in a Heineken Cup match of this intensity."
In the backs, four players will be making their debuts. Stand-off Ryan Lamb, centres Jack Adams and Anthony Allen (all 20) and wing Mark Foster will gain their first experience in the competition.
Full-back Iain Balshaw has made 19 appearances in the competition for Bath, while Peter Richards has played for four different clubs in the European Cup.
"Being part of this tournament was a reward for what we managed to achieve last season," Ryan said of his Challenge Cup victors. "I don't think there is any greater stage than Lansdowne Road in the Heineken Cup and it will obviously be a huge test for this team.
"In my opinion they produced the performance of the tournament last year when they beat Toulouse away and we obviously have a team with a low percentage of players who have been exposed to this competition.
"It will be an enormous challenge but I am sure we are capable of winning - I am sure we will learn more about this group on Saturday - it's a fantastic opportunity for everybody."
GLOUCESTER v (Leinster, tomorrow, Lansdowne Road, 5.30pm): Iain Balshaw; James Bailey, Jack Adams, Anthony Allen, Mark Foster; Ryan Lamb, Peter Richards; Christian Califano, Mefin Davies, Carlos Nieto, Marco Bortolami (capt), Adam Eustace, Peter Buxton, Andy Hazell, James Forrester. Reps: Nick Wood, Olivier Azam, Jonathan Pendlebury, Jake Boer, Rory Lawson, Ludovic Mercier, Rudi Keil.
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