Looking at the defensive components of his fWAR by Month, pre-trade deadline he was -15.9 in 65 games which is like -1.6 fWAR because 10 runs for 1 win is the rule of thumb. Most of those games were in left, though he did play less LF and more 1B in July and his defensive portion is much better in July.
So -1.6 fWAR in LF in 65 games defense only.
-3.2 in 18 games in August/September, which is -0.3 fWAR in 18 games defense only. Pro-rated to 65 games that would be -1.1.
So in 65 games his WAR would be 0.5 better at 1st, over 162 games that's 1.2 better WAR just from playing 1st base instead of left field.
It's not perfect, because some of those pre-August games he was playing some 1st base, mainly in July when the defensive portion wasn't as bad, so I actually think he could gain more than 1.2 WAR over 162, but it's pretty close.