Ruben Amorim substituted Alejandro Garnacho during Manchester United's Europa League draw with Real Sociedad as he was becoming "late" to defend.
Garnacho had several shots at goal but has now gone 22 games without scoring, having tallied eight in his first 20 appearances of the season.
The winger made way for Christian Eriksen in the 78th minute at the Reale Arena, seven minutes after Mikel Oyarzabal equalised and sparked Sociedad's best attacking spell.
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"I felt he (Garnacho) was tired," Amorim explained, "and you see it because we did a lot of transitions in the second half because sometimes he was late to defend. And sometimes he’s in a good position to transition but I felt he was tired to recover and do the same things. I look at the game as a whole and not just in transitions."
United had 13 attempts at goal but only five were on target and Amorim rued their failure to go 2-0 up in a tie they were controlling competently until Bruno Fernandes was penalised for handball following an intervention by the Video Assistant Referee.
"We are creating chances but sometimes you don’t see it in the xG (expected goals) because we manage not to shoot but the great opportunity is there and you can see it," Amorim added. "We have transitions, we have situations and sometimes we are missing something.
"That can change something, especially in these kind of games that we need goals that can make a difference."
Amorim was pleased for goalscorer Joshua Zirkzee, who slotted in his sixth for United four days after he failed with the decisive kick in the penalty shootout defeat to Fulham in the FA Cup fifth round.
"He deserved it because he is working really well," Amorim enthused. "He’s improving a lot of things in his game, he deserved this moment and he was really important for us today."
