Contepomi suffered bruised ribs in the Pumas' 13-9 defeat to England in their World Cup opener last weekend and will miss Saturday's game as Argentina look to bounce back immediately.
Argentina also lost former Harlequins and London Irish centre Gonzalo Tiesi for the tournament with ligament damage, prompting coach Santiago Phelan into a back-line reshuffle.
Santiago Fernandez takes over at fly-half and Marcelo Bosch at outside centre, while former Sale number eight Lobbe assumes the captaincy responsibilities for the seventh time in his career.
"He (Contepomi) will be missed. He's our star, the leader and the captain who can make a difference," said Lobbe.
"But we believe we have a strong squad.
"It brings me great happiness (to be captain) and it is a really nice responsibility, but what always makes me glad is the opportunity to pull on this Pumas jersey."
While Argentina fell just short in a physical game against England, Romania led Scotland with seven remaining before conceding two late tries.
"They have a really good pack and a good defence - we saw that against Scotland," said Lobbe.
"We'll have the same approach - a great defence, keep our tackling low, and try to stop them at lineouts and rolling mauls.
"Everything we do is for the team - 110% is the only option. We will put our bodies on the line and our hearts on the pitch."
Pumas veteran hooker Mario Ledesma, who is older than both head coaches in this game, will break an Argentina record when he plays his 15th World Cup match.
Romania coach Romeo Gontineac has made three changes from the Scotland performance, including a switch for Tiberius Ionut Dimofte from inside centre to fly-half.
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