The design is the visual features of the entirety of the ACHILLES TENDON PROTECTOR shown in the drawings, whether those features are features of one of shape or configuration or are a combination of any of these features. Drawings of the design are included, as follows: FIG. 1 is a front perspective view of an Achilles tendon protector according to a first variant. FIG. 2 is a right side view of the Achilles tendon protector shown in FIG. 1. FIG. 3 is a front view of the Achilles tendon protector shown in FIG. 1. FIG. 4 is a left side view of the Achilles tendon protector shown in FIG. 1. FIG. 5 is a rear view of the Achilles tendon protector shown in FIG. 1. FIG. 6 is a top view of the Achilles tendon protector shown in FIG. 1. FIG. 7 is a bottom view of the Achilles tendon protector shown in FIG. 1. FIG. 8 is a front perspective view of an Achilles tendon protector according to a second variant, which is identical to the first variant except for omission of the foot strap. FIG. 9 is a front perspective view of an Achilles tendon protector according to a third variant. FIG. 10 is a right side view of the Achilles tendon protector shown in FIG. 9. FIG. 11 is a front view of the Achilles tendon protector shown in FIG. 9. FIG. 12 is a left side view of the Achilles tendon protector shown in FIG. 9. FIG. 13 is a rear view of the Achilles tendon protector shown in FIG. 9. FIG. 14 is a top view of the Achilles tendon protector shown in FIG. 9. FIG. 15 is a bottom view of the Achilles tendon protector shown in FIG. 9. FIG. 16 is a front perspective view of an Achilles tendon protector according to a fourth variant, which is identical to the third variant except for the addition of a foot strap. FIG. 17 is a front perspective view of an Achilles tendon protector according to a fifth variant. FIG. 18 is a right side view of the Achilles tendon protector shown in FIG. 17. FIG. 19 is a front view of the Achilles tendon protector shown in FIG. 17. FIG. 20 is a left side view of the Achilles tendon protector shown in FIG. 17. FIG. 21 is a rear view of the Achilles tendon protector shown in FIG. 17. FIG. 22 is a top view of the Achilles tendon protector shown in FIG. 17. FIG. 23 is a bottom view of the Achilles tendon protector shown in FIG. 17. FIG. 24 is a front perspective view of an Achilles tendon protector according to a sixth variant, which is identical to the fifth variant except for the addition of a foot strap. FIG. 25 is a front perspective view of an Achilles tendon protector according to a seventh variant. FIG. 26 is a right side view of the Achilles tendon protector shown in FIG. 25. FIG. 27 is a front view of the Achilles tendon protector shown in FIG. 25. FIG. 28 is a left side view of the Achilles tendon protector shown in FIG. 25. FIG. 29 is a rear view of the Achilles tendon protector shown in FIG. 25. FIG. 30 is a top view of the Achilles tendon protector shown in FIG. 25. FIG. 31 is a bottom view of the Achilles tendon protector shown in FIG. 25. FIG. 32 is a front perspective view of an Achilles tendon protector according to an eighth variant, which is identical to the seventh variant except for omission of the foot strap. FIG. 33 is a front perspective view of an Achilles tendon protector according to a ninth variant. FIG. 34 is a right side view of the Achilles tendon protector shown in FIG. 33. FIG. 35 is a front view of the Achilles tendon protector shown in FIG. 33. FIG. 36 is a left side view of the Achilles tendon protector shown in FIG. 33. FIG. 37 is a rear view of the Achilles tendon protector shown in FIG. 33. FIG. 38 is a top view of the Achilles tendon protector shown in FIG. 33. FIG. 39 is a bottom view of the Achilles tendon protector shown in FIG. 33. FIG. 40 is a front perspective view of an Achilles tendon protector according to a tenth variant, which is identical to the ninth variant except for the addition of a foot strap.