Renault's power unit paled in comparison to that supplied by Mercedes, whose works team dominated a world championship won by British driver Lewis Hamilton.
Grosjean drove a Renault-powered Lotus in 2014, but will compete in a Mercedes-backed car in 2015 after the team opted to switch suppliers.
And Grosjean expects Renault to make strides in the coming year.
"They [Renault] will improve. They have always been able to make good engines. They messed up this year," Grosjean said.
"The engine was off at the start of the year, but they improved as much as they could.
"That made it hard work, but Mercedes has done a really amazing job this year, although they have room to improve and people will continue chasing them as they will still be ahead."
The 28-year-old Grosjean scored just eight points last season and feels that Lotus need to recognise problems earlier in the testing process in order to enjoy a better campaign next time out.
"I think mainly in the first test we need to understand everything we understood by August this year," he added.
"We need to know what the car is doing and its characteristics and straightaway know where we can improve, unlike this year where we didn't know where we could improve in testing.
"Then we knew our cooling was bad and we spent a lot of time on it, and then the windtunnel was not showing the problems we were having.
"We know we can understand the car much quicker now and know which areas we can focus on straightaway."