Let's pump the brakes on the 'one of the best players in the league' talk. The metrics rated him as an elite fielder last year, but outfielders almost never put up that kind of defensive value consistently. We don't know enough to say that he is Andruw Jones, pretty much the only player to do it.
Look at Kevin Kiermaier's UZR/150 for a sample of 6 years
21.3
8.2
16.4
9.8
30.7
12.1
Is he that inconsistent in the field? He isn't, but the metrics are. I'm confident that the average tells us what kind of fielder he is, but one year doesn't always tell the full story. Even Varsho's number jumped from 13.9 in his first year to 23.2 in his second.
In left field with an average bat, he's probably a 3 to 3.5 win player if you allow for the fielding metrics to stabilize. That's just fine, but the projection systems think he's in line for improvement at the plate and so do the Blue Jays. That's why they made the trade.