Recruited by the Eagles as a restricted free agent, Ellis gathered 21 disposals, running off half-back, to help set-up the Eagles' 66-point half-time lead.
"We're really happy with his game," Simpson said.
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"I'm not quite sure if he'll be allowed the space he had today if it was a round one game but it's good for his confidence. He didn't play a full game but he had impact."
The former Hawk was subbed out just after half-time as part of his ongoing management.
Ellis badly tore his calf playing for Hawthorn in the 2012 Grand Final and as a result he managed just two matches for the Hawks in 2013 before following Simpson to West Coast.
"He's on a specific program that gives him an amount of minutes for this week, next week, and the week after," Simpson said.
"We want to build him up to obviously being able to play a full game. I'm not sure of his game time today but it would have been just over a half, which is what we wanted to get out of him. He finds a way to get hurt, X."
Although delighted with his performance, Simpson was keen to temper expectations on the 25-year-old.
He's played just 86 matches in seven seasons of senior football. However, Simpson believes the quality he displayed against Fremantle on Tuesday should not surprise anyone.
"It's the reason why we got him over," Simpson said.
"He's a No.3 draft pick. He's got good composure, he's talented, and he's got great character. We just need to get that body right."
Simpson was also impressed with Sharrod Wellingham's performance.
The former Magpie, in his second year at West Coast, collected 14 disposals and kicked three goals against Fremantle having overcome a bone-stress injury in his lower leg that had severely hampered his pre-season.
Wellingham had a horror first season at West Coast, playing just 10 games but his new coach was happy to see him out on the park.
"It's good to have him out there," Simpson said.
"He's another player who is prone to injury. When he's fit and firing, he's a talent.
"He's a quality player. No doubt."
