Oddly the Laws make no reference to whether the action was deliberate they simply require the offence to be careless, reckless or using excessive force.
“Careless” means that the player has shown a lack of attention or consideration when making a challenge or that he acted without precaution.
• No further disciplinary sanction is needed if a foul is judged to be careless
“Reckless” means that the player has acted with complete disregard to the danger to, or consequences for, his opponent.
• A player who plays in a reckless manner must be cautioned
“Using excessive force” means that the player has far exceeded the necessary use of force and is in danger of injuring his opponent.
• A player who uses excessive force must be sent off
The difference between the last two is completely subjective unless you think that the fact a serious injury has occurred proves excessive force was used.
And a pedant might observe that if you deliberately trip someone up in a gentle way you can't be acting 'carelessly' so no foul has been committed!