If we had good intel on the amount of time Moses is out (3-4 more weeks), then Field would probably be a good option. Unfortunately, its all up in the air at the moment with Moses even saying he won't rush back, but that could be him being cagey so opposition teams don't prepare for him.
Gutherson's spike in scores is mainly because of his taking on more of the play-making duties and getting early ball from Field who is obviously less experienced. You look at the number changes in Gutherson's stats with Moses on/off the field:
With Moses (6 rounds): 2 tries, 1 try assist, 4 linebreaks, 3 linebreak assists
Without Moses (2 rounds: 0 tries, 5 try assists, 3 linebreaks, 6 linebreak assists!
Last match where Field scored 80, he got a try and a linebreak but more importantly hasn't been credited with any Try Contribution, Linebreak Assists or Try Assists. This screams trap to me, as when Parramatta come up against better defensive units, Field will have less and less ball, with Gutherson and Dylan Brown being favoured to handle the ball under pressure.
By the way, I'm not trying to discourage people from grabbing Gutherson, just pointing out the uncertainty over him continuing the heavy scoring when Moses is back. His draw over the next 5-6 weeks looks exceptionally tempting, but even against the Bulldogs and Titans, he scored sub-40 because Dylan Brown and Moses stole the majority of the attacking points.
TLDR: If you already have Gutherson, keep him and hope he averages 70+ over the course of the season. If you don't, weigh up whether you can find value elsewhere before chasing Gutherson's points.