With Button's contract coming to an end this season, McLaren is facing a tough call about whether to retain the Briton alongside Fernando Alonso or take rising youngster Vandoorne.

McLaren has already told rival teams to keep their hands off Vandoorne, who has an option in his contract that will lock him into the Woking-based outfit if they want him.

Boullier said earlier this year that a decision about drivers would be taken in September, and he is well aware about how tough a call he and senior management must make.

But speaking to Motorsport.com about the situation, Boullier made it clear that he felt it important McLaren made the call for rational reasons.

"We have to make a decision which will be the McLaren strategy," explained Boullier. "After that there are some pros and cons but Ron [Dennis], myself and the shareholders, we cannot be emotional.

"That is what drives the fans and the sponsors maybe, but we cannot be emotional. We need to be rational about what we want to achieve at McLaren this year, next year and the following years."

When asked if McLaren favoured a long-term strategy that would favour Vandoorne, or short-term potential with Button, Boullier said: "We need to have both.

"What McLaren went through in the last two years was a deep change, and having Honda onboard now as a new player in F1, we can play it medium and long term. So we will see."

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