American driver Rossi was part of the Caterham fold from 2011 until last month, just after the team was sold to an investment group by founder Tony Fernandes.
The 22-year-old had four grand prix Friday practice outings with the team in that period and also made four F1 test appearances, alongside Caterham-backed campaigns in Formula Renault 3.5 and GP2.
Rossi admitted he feared he would rapidly vanish from the F1 radar following his Caterham exit before he found a new home at Marussia.
"It was critical. Mainly because I didn't want to be seen as being out of the Formula 1 gang, so to get the deal done so quickly with Marussia before the summer break was critical," he told AUTOSPORT.
"I thought I had everything going for me and if I didn't get this deal done there may have been a loss of interest and loss of faith.
"So the fact this has happened, in my opinion, resets all the negativity, not only over the past few weeks, but the past couple of years.
"It's a new chapter for me and I'm looking forward to getting into the car at Spa and showing what I am capable of.
"I think the future looks very bright at Marussia."
Although his schedule of practice drives with Marussia is not yet determined, Rossi said it will "certainly be more than the two I was due to have with Caterham" and thinks his new deal gives him a better chance of showcasing his ability.
"To be honest, I have not been too pleased with the past two years of my career," he said.
"I feel it could have gone much better, but circumstances prevented us from getting the results that I think I deserved.
"The teams saw the potential in me and saw I was on the market, so it was a relief [to get the Marussia role].
"But until you get that phone call, there is kind of that unknown in your own abilities, so I feel very grateful."
Subscribe to Autosport Plus
Register for free