Magnussen is moving to the American team from Renault, whose Cyril Abiteboul this week said that the Danish driver "stood still" while wearing yellow last season.
"Personally, I was disappointed in Magnussen, for his performances and his behaviour," the Renault boss told Auto Hebdo.
"Kevin is a talented driver, but he lacks discipline. Also, he tries all too often to make excuses."
When asked about Abiteboul's attack, Haas team boss Steiner dismissed the remarks.
"Kevin is a respected race car driver, and I must emphasise that," he is quoted as saying by Danish newspaper Ekstra Bladet. "We are all looking forward to working with him."
Magnussen accrued seven points for Renault in 2016.