The scout asked former major league player Chris Gwynn to provide some instruction to Collier.

He gained broader attention after working out at the Major League Baseball Urban Youth Academy.

Jim Callis of Baseball America commented on Philadelphia's early draft choices, referring to Collier and three other draftees as "classic high-upside, high-risk players.

The next season, Collier split the year between the Low-A Williamsport Crosscutters of the New York–Penn League and the Single-A Lakewood BlueClaws, collecting had 20 stolen bases and eight triples, but recording a low .221 batting average.

[8] He spent the 2013 season in Double-A with the Reading Fightin Phils, slashing .222/.310/.348 with 8 home runs, 36 RBI, and 17 stolen bases.

[10] Collier hit .240/.318/.408 with 9 home runs and 29 RBI in 86 games in Double-A and elected free agency after the year.

[11] He was assigned to the Triple-A Syracuse Chiefs to begin the 2016 season, and also spent time with the Double-A Harrisburg Senators, batting .251/.311/.373 with 4 home runs and 31 RBI between the two teams.

[12] In 2017, Collier split the year between Syracuse, Harrisburg, and the Low-A Auburn Doubledays, hitting .281/.361/.482 with 8 home runs and 24 RBI in 68 games between the three teams.