The motivation for the move is relatively obvious, at least from the Mariners perspective. The Mariners could really use some starting pitching depth, or pitching depth in general. This is more a function of an inordinate amount of injuries to the pitching staff that leave it dangerously thin than poor planning from the M's front office. Felix Hernandez and Wade Miley are on the DL from the starting rotation, and news just broke that Taijuan Walker is getting an MRI on his foot. In the bullpen Charlie Furbush, Ryan Cook, and Evan Scribner are all yet to make their season debuts. No team suffers that many arm injuries without consequences.
All in all, the Mariners have almost half a pitching staff with significant injury concerns. Furthermore, Jerry Dipoto had to work in the offseason to build pitching depth that was largely ignored and decimated by the end of the Zduriencik era. This is approaching (if not arriving) at a worst-case scenario for how the year could have unfolded for the pitching staff.
So, on some level, Zach Lee fills a need that developed with the way the season unfolded. However, Dipoto announced that Lee will report to AAA Tacoma, meaning he does not fill an immediate need on the 25-man roster. He also does not fit the profile of a typical stop-gap player. There is a little more to this deal beneath the surface.
Zach Lee, not all that long ago, was an interesting prospect. The Dodgers drafted him in the first round straight out of high school and backed up a Brinks truck to the tune of $5.25 million to sign him away from playing quarterback at LSU. He signed, and the rest is history to some degree. Lee steadily climbed the minor league chain before stalling in AAA where has neither flourished nor floundered. Lee now profiles as a durable innings-eater. He doesn't miss a ton of AAA bats, which suggests he will pitch to even more contact in the majors. However, Lee also does not walk that many batters. One might say he controls the zone. I would sandwich Lee somewhere between Blake Beavan and Doug Fister, for those of us who enjoy comps.
Chris Taylor is sort of a known commodity. I saw him play several times in Tacoma and am sorry to see him go. He has hit too much in AAA to stay as anemic in the majors as he has. Taylor has gap power and just needs to cut down on strikeouts to stick and be a serviceable MLB infielder. He has no amazing tools but relatively few weaknesses as well. Taylor is simply a solid, fundamentally sound baseball player.
This trade is about more than immediate depth though. Zach Lee is almost exactly a year younger than Chris Taylor with about half a season less of MLB experience. Maybe more importantly, only one of his option years is used up. Lee might never become a frontline starter in the M's rotation, but he brings several years of minimal cost-controlled certainty to the team.
If the Mariners really just needed another arm they could have probably found a veteran for cheap. For instance, Mat Latos is in DFA limbo right now after the White Sox removed him from their roster. There were ways to add arms that involved holding on to a guy like Chris Taylor.
So this move isn't just about the injuries to Mariner arms. Zach Lee fits the Dipoto mold. He throws lots of strikes and comes to Seattle with lots of cheap, team-controlled years. I would hazard to guess that Lee has been on the Dipoto's radar for some time but Dipoto was holding out for a better deal that did not involve Chris Taylor. However, the M's current situation might have convinced Dipoto to pull the trigger.
In general, I would rather be the team acquiring a shortstop for a pitcher. However, Lee's extra team-controlled years and the immediate need for pitching depth over infield depth have to be considered. I think the Mariners gave up a better player than they received but they might have acquired the better asset. Lee won't just add depth this season; he adds depth for many seasons to come and he is young enough to still develop a bit.