According to NY Times reporter James Wagner, Juan Lagares is working this winter with Craig Wallenbrock, who is the hitting instructor credited with changing J.D. Martinez's swing.
Prior to working with Wallenbrock, Martinez hit .251 with a .300 OBP and just 24 home runs in nearly 900 at bats, during which he bounced between the Astros and their upper-level affiliates.
The season after working with Wallenback, at which time Martinez was 26 year sold, he hit .315 with 23 home runs and 76 RBI, while striking out less and getting on base more than earlier in his career.
In his subsequent three seasons, Martinez has hit 105 home runs and enters this winter a top free agent corner outfielder reportedly seeking a $200 million contract.