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The Japanese forward is likely to miss the weekend's game after sustaining an ankle injury at West Brom. He will see a speicalist on Friday to determine the severity of the issue.
That potentially opens a spot in attack, with Mowbray largely naming the same team for most of the campaign. Six of his team have started all 11 games so far.
The head coach insists he's comfortable with the squad's depth and wants those on the fringes to grab their chance.
"Every club lives and dies by their strength," Mowbray insisted.
"Every club will get injuries and suspensions as the season unfolds. The players' responsibility is to be ready, regardless of whether they've played for a few weeks in the team or had not enough minutes off the bench or whatever it might be.
"They have to be ready and if they're going to start this weekend, I have to manage the minutes of the players that I think can do 90 or the ones I think can maybe only get 60 or 75 out of.
"The guys coming on in modern day football are called the finishers. They have to finish the game, either score to help us get a victory or, at times, to help the team work really hard and stop the opposition scoring to get back in it.
"I'm relaxed about the team and the quality that we've got as a squad, whilst the number of injuries are where they are. Obviously, the Morishita situation, if he doesn't make it, is frustrating for us because he's such an important player for us.
"If it's a defeat on the weekend, I won't be making excuses about squad depth and losing Morishita, for instance."
(Image: CameraSport - Andrew Kearns)
Despite his continuity in selection, Mowbray hasn't had any knocks on his office door or slumped shoulders.
"No, I think I've always tried to foster a squad feeling," he insisted.
"The lads who don't even make the matchday squad, but travel, are still hugely important.
"Scotty Wharton is now back in and around it, yet I left him out of the matchday squad.
"He came in the travelling squad and he's an amazingly good professional, encouraging and around everybody and got his kit on and did the extra running at the end and parts of the warm-up.
I've always said that, the more you moan and the more you soak that you don't get on or you get brought off or you don't be part of the matchday squad, you're actually pushing yourself further away from the starting team.
"I want good human beings, good professional footballers who understand why they're not in the team. They might be coming back from injury; they might not have played particularly well in the last game or two.
"Somebody else deserves a chance. You just get on with it and be supportive of your teammates. If we need you, we'll put you on and hopefully you can do what you do, help us defend a lead or go and score a goal."
"That's how I judge them really. I think they know that."