Grosjean endured a tough campaign last year as he was involved in a number of first-corner incidents, culminating in him instigating a pile-up in the Belgian Grand Prix that led to a one-race ban.
The Frenchman was ultimately forced to seek guidance from a sports psychologist to help him overcome his troubles at the start of races.
Retained by Lotus at the end of the campaign as they showed their faith in the 27-year-old, Grosjean then suffered with car issues over the first three grands prix of this season.
That resulted in the Enstone-based marque changing the chassis ahead of the last race in Bahrain, resulting in a difference described by Grosjean as "day and night".
Grosjean's reward was his first podium finish for 12 races as he was third behind winner Sebastian Vettel in his Red Bull and Lotus team-mate Kimi Raikkonen.
"For me, he is starting his season now," said Boullier.
Asked about the difference the sports psychologist had made, Boullier replied: "He had a difficult end to last season, so we had to provide him with every means to ensure he could rebuild himself.
"If you look at the season he's had so far, except with some issues over speed, he's not had any problems.
"Now he's making good starts, having good opening laps, making good overtaking manoeuvres. He's okay now."
Boullier confirmed the changes to the car also played their part.
"It was part of the review we went through with Romain, not only with regard to the chassis, but a lot of little details," added Boullier.
"We had to understand why things weren't working, and in the end the team did a very good job.
"We discovered a couple of things to make Romain happy with his car, and when he started to enjoy his driving again, it was a different world.
"It was nice to see him smiling again."