Mariners are about to welcome back final puzzle piece for their pennant chase

Minnesota Twins v Seattle Mariners | Stephen Brashear/GettyImagesIt's rare for teams to add impactful players after the trade deadline. Usually, the only external players that are available are those who have been released, designated for assignment, or were free agents that never found a home to begin with. Internally, rosters for playoff hopefuls get upgraded in two ways: late call-ups and returns from the injured list.

For the Seattle Mariners, the latter could hold the secret to getting another leg up over the competition.

There are several key players who have officially started rehab assignments and they've all been going well. Luke Raley has posted a 128 wRC+ in his second rehab stint…