Magnussen, 23, has served as McLaren's reserve driver this year, having lost his seat at the team despite an impressive maiden season in 2014 that included a podium on debut in Australia and a 9-10 qualifying record against 2009 world champion Jenson Button.
With Magnussen saying earlier this year that he would not settle for another season on the sidelines in 2016, Haas made contact with the Dane - but team boss Gene Haas said the fact they were able to persuade Grosjean to join from Lotus ultimately scuppered the deal.
"We talked to Magnussen, who was a very good candidate," Haas told Formula1.com.
"He was very well respected by the McLaren team. In fact if Romain had declined we would have taken Magnussen as the primary seat."
Haas also revealed that Haas had been in deep talks with another driver on the current grid, but after a month of negotiations the unnamed star eventually decided to stay with their current team.
McLaren confirmed in mid-October that they will not retain Magnussen in 2016, and with Haas announcing Esteban Gutierrez as their second driver in Mexico, Marussia are now the only team with likely vacancies for 2016.
Look out for the full, exclusive interview with Gene Haas on Formula1.com later this week.