The Gabon international is Saints' record signing after joining for £16m from Juventus last month.
Romeu said: “He's been very good. He's a great signing. He's a very strong player. He helps the team a lot. He's defensively strong and also can drive forward and create chances.
“It's very good to have him, so hopefully we can keep helping him and he can get better. Then, together, we can make a stronger team.
"Mario was playing in Italy but I hadn't seen him play before now. I'm surprised the way he is and he's a great player."