Walker, 21, made his season debut with Seattle in the same stadium in which he made his first Major League start. In that game, last Aug. 30, Walker picked up the victory while allowing just one earned run in five innings against Houston.
Walker pitched against the Astros twice in his three starts with the Mariners late last season, going 1-0 with a 3.60 ERA.
The hard-throwing youngster was expected to open this year in the Mariners' rotation, but Walker missed most of the spring with a sore right shoulder and spent time early this season on the disabled list before working his way through a pair of Minor League rehab assignments and then a stint with Tacoma.
Walker, the No. 5 overall prospect in baseball according to MLB.com, went 2-1 with a 4.11 ERA in six starts with the Rainiers and was named the Pacific Coast League Pitcher of the Week on Monday for his four-hit shutout performance in his Tacoma finale against Oklahoma City last Tuesday.
Romero, 25, appeared in 51 games and hit .196 with 18 runs, six doubles, two triples, three home runs and 11 RBIs. Romero hit .263 (15-for-57) on the road and .154 (14-for-91) at Safeco Field. McClendon said the youngster would benefit from playing every day again in Tacoma.
"It gives him a chance to go down and get some at-bats," McClendon said. "I think this young man has a bright future, but I know -- having lived it and having done it -- it's the hardest job in baseball to play once or twice a week and expect to be productive. If he gets back here, he'll be a better player for us as a result."
The Mariners chose to keep right-handed reliever Brandon Maurer and go with an eight-man bullpen for now, which leaves just three position players on the bench each night. But McClendon said Maurer's impressive performance -- three scoreless innings with six strikeouts in two appearances - warranted the move.
"There are a couple factors that played into it," McClendon said. "No. 1, our pitching has been our foundation, and this gives us a chance to protect it a little and shorten up ballgames. We all know offensively we're challenged a little, so it gives us a chance to shorten games when we can and protect guys and not overuse anybody. And this guy was pretty impressive coming out of the bullpen. It's very hard to ignore what he did two times coming out of the bullpen, so we'd like to see more of him."
Even without Romero, the Mariners have five outfielders with Dustin Ackley, James Jones, Michael Saunders, Endy Chavez and Cole Gillespie, while Willie Bloomquist can also play there if needed.
"Bloomy can play all over the place, and that obviously gives you a little more flexibility," McClendon said. "Listen, it's not like we've had this tremendous bench that we've run through every game. That factored in as well. In the American League, you have a DH. You don't pinch-hit that much and use your bench that much. And we don't play the NL until Aug. 18, so it gives us a chance to give us a little more run with our pitching."