CARLTON has received a welcome boost after its tough start to the season with emerging defender Sam Docherty re-signing to the end of 2018.

Docherty had been contracted to the end of 2015 with a trigger clause for 2016, but the 21-year-old recently signed a new three-year deal with the Blues.

Carlton football manager Andrew McKay told AFL.com.au that Docherty was a key part of the Blues' future as they prepare to embark on a widely publicised list rebuild.

"We see Sam as 200-plus gamer here, we him staying here for the long haul and for him to put pen to paper for three years is really positive for us," McKay said.

"We're looking for some big improvement for the team between now and then and Sam will be a part of our journey in the next few years.

"Sam's a quality young player and a quality person too."

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Docherty's has spent nearly all of his first two seasons at Ikon Park in defence but McKay said he had the tools to eventually move into the midfield.

"At the moment he's playing really good footy in defence. He floated through the midfield briefly the game before last and he's got the potential to go there more often as his game matures," McKay said.

Docherty has made a strong start to 2015.

After five rounds, he leads Carlton in total kicks (14.6 a game) and rebound 50s (5.2), is second in marks (6.2) and equal second in uncontested possessions (15).

He is also fourth in total disposals (21.2 a game), fourth in one-percenters (4.2) and equal eighth in tackles (2.8).

Docherty finished ninth in the Blues' 2014 best and fairest award in his first season at the club despite missing the first six rounds because of a pre-season quadriceps injury.

The former Gippsland Power player was drafted at pick No.12 in the 2011 national draft by the Brisbane Lions, where he played 13 games in two seasons.

Docherty joined Carlton in the 2013 trade period in exchange for pick No.33, which the Lions used to select Tom Cutler.