Lotus Renault team principal Eric Boullier has admitted that Nick Heidfeld's inability to establish himself as the natural leader of the Enstone squad eventually led to him being replaced by Bruno Senna at this weekend's Belgian Grand Prix.

The Brazilian ran a Friday practice session in place of Heidfeld at the Hungarian Grand Prix, but the outing was seen largely as a means of fulfilling a sponsorship requirement and potentially refreshing Senna ahead of a late-season race opportunity. However, with Renault's season in danger of stalling after being passed by Mercedes as the fourth-best team in the championship, Boullier decided to promote Senna to a full race seat for the final eight rounds. Despite being two points ahead of team-mate Vitaly Petrov in the standings, Heidfeld - who was hired to replace the injured Robert Kubica at the start of the season - was the one to get the axe.

"We reviewed our performance and level of motivation, a number of things over the summer, and we chose a new direction," the Frenchman told Associated Press, "There was also an opportunity to assess Bruno as a driver, and this is why we have moved on."

Stressing that the decision had 'nothing to do with money', after critics claimed that Senna was bringing greater amounts of sponsorship to a cash-strapped team, Boullier also claimed that dropping Heidfeld was only partly to do with the German's pace.

"Every session, every weekend, the media have jumped on me asking why Vitaly is faster than Nick, [and it's true that] I was not very happy with his speed and overall performance as an experienced driver," Boullier acknowledged, "Nick is a nice guy, but leadership didn't work. When you are slower than Vitaly most of the time, it's difficult for him to push the team and set up himself as team leader.

"The story is always the same. There are links with Brazil, because Genii deals in a billion-dollar fund there, but that has nothing to do with Bruno. He is the third driver and, when you change a driver, it is only natural to promote the third driver."

Heidfeld did not cop all the blame for his sacking, with Boullier accepting that the team had fallen away in its programme with the R31, but the Frenchman insisted that action had to be taken to prevent the season from tailing off as well.

"I admit the car is not good enough, we have not developed it well enough, so we have made mistakes as well," he conceded, "When you have a negative spiral, it's complicated to stop it, so I had to change something within the team, something with the drivers to shake up and wake up everybody. It's a risky decision but, if it's the only chance to get back to the front, then so be it. Of course, I get the blame when it does not work, but it's my job to make decisions."

Boullier also confirmed that there was a chance that Heidfeld could return to the cockpit after the Italian Grand Prix, but only because the German had refused to settle out of court and was pursuing legal action in a bid to regain his seat. With Renault having won an initial hearing to allow it to field Senna at Spa and Monza, however, it will take something approaching a U-turn on 19 September for Heidfeld to succeed.

"The decision was for Bruno to replace Nick until the end of the season," Boullier confirmed, "but, because there is now another hearing, anything is possible. They have a different interpretation of the contract to me, so opinions vary. Nick will not negotiate a different agreement, so we were already in court, but everything he claimed was rejected."

The team is not expecting immediate miracles from Senna, with Boullier accepting that it could take until the Singapore Grand Prix for the Brazilian to be able to match Petrov for pace, but even that may not be enough to keep the former HRT pilot in the Renault line-up for 2012, if Kubica proves his fitness.

"Robert needs a final operation in the next few weeks," Boullier revealed, "and then he can begin to recover. He needs to regain his driving experience but, when he is ready, we'll ready a 2009 car for him to test."