Henderson has been world class since the turn of the year, was good earlier in the season too but I think he's starting to be in contention due to his recent performances (similar to a boxer producing a late flurry in the last minute to win the round)
I think consistently Van Dijk has been at a top, top level all season. As has Mane, so I think they'd deserve to be ahead of him in that regard.
Personally I'd give it to Trent. He's been absolutely magnificent this season and is absolutely integral to our squad. Only issue is that he's nailed on for the Young Player of the Year award & it's very rare that they give players both (I think Ronaldo & Andy Grey are the only two who have done it in the past). Think if he was 26/27 he'd be right up there as a favourite to pick it up