TORONTO -- The "workhorse" moniker doesn’t get thrown around often these days. Luis Castillo is one of the select few who have certainly earned the title.

The Mariners' Opening Day starter has pitched thirty or more games three times in his career, he earned American League Cy Young votes for Seattle in 2023, and he's about as consistent as they come.

But in the early days of the 2024 season, the consistency and heavy innings have eluded Castillo. He has yet to pitch six full frames this year and entered his third outing with a 6.75 ERA. In Seattle's 5-2 loss to the Blue Jays at Rogers Centre on Monday night, the righty’s search for his workhorse self continued.

“Luis is certainly capable,” manager Scott Servais said. “He wouldn't be the pitcher he is in this league unless he had the ability to go and finish off hitters. He's just struggling with it right now.”

For a second straight outing, Castillo looked to be back on track as the game began. In his last start, against the Guardians, the Seattle hurler retired eight of his first 10 batters, pitching three scoreless innings before things unraveled. On Monday against the Blue Jays, Castillo retired the top of Toronto’s order with ease -- three straight routine groundouts to get out of the first inning on just eight pitches.

In that opening frame, Castillo was living on the edges, inducing soft contact and cruising. It was exactly what the righty’s been searching for, as he’s been either in the heart of the zone or missing by too much, resulting in plenty of hits and walks.

“His stuff has been fine, it’s more about the execution,” Servais said before Monday’s game. “Sometimes the ball has been in the middle of the plate at the wrong times, to the wrong hitters.”

But after the first inning, the walks and mashable middle pitches returned. After allowing the first three batters to reach base in the third frame, Castillo faced Toronto's Davis Schneider with the bases full.

After getting to two strikes, Castillo put a slider right where he wanted. The wide-breaking pitch slid out of the strike zone and away from Schneider, but just not quite far enough. Schneider slashed out at the pitch, connecting on the breaking ball and sending a looping liner to the outfield grass. As the hit landed on turf, two Toronto runners rounded third and slid across the plate, putting the Mariners down 3-0 early.

The Blue Jays added one more run in the fourth before Castillo’s day ended after five frames and 101 pitches. Four runs across five innings is far from a disaster start, but for a guy like Castillo, who pitched at least six innings 23 times last year, it's not up to his standard. And with the Mariners' offense scuffling to start the season (they recorded just two runs in Monday's loss) it wasn't enough.

“I think the key is to just attack the batter -- that’s going to be the key for me to go longer [into games],” Castillo said via team interpreter Freddy Llanos. “And also try to get the batters to get soft contact to get the outs early.”

You don’t have to look very far to see what Castillo can be when he’s on. Toronto’s Rogers Centre -- the venue for Castillo’s outing on Monday -- is the same building in which he carved through 7 1/3 scoreless frames in Game 1 of the 2022 AL Wild Card Series, looking utterly unhittable in that Mariners win.

When he’s dialed in, Castillo is that ace. When he’s living on the edges and in complete control, he can pitch deep into games. So far this season, Castillo hasn’t yet been the workhorse.

“I think if anybody’s got their top of the rotation guy struggling like that, you need him back,” Servais said. “There’s no question about it. He’s certainly capable of doing that.”