The Spaniard was left out of Manchester United's squad completely at the weekend when the Red Devils travelled to Sunderland.
Mourinho stated he had a hip problem but reports elsewhere suggest the Portuguese has been concerned De Gea's focus could be distracted with Real Madrid chasing his signature.
David De Gea as benched for Manchester United's draw with Anderlecht Manchester United player ratings against Anderlecht Thu, April 13, 2017
Express Sport rates the Manchester United players' performances during their 1-1 draw with Anderlecht
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Which Manchester United players impressed during their 1-1 draw with Anderlecht?
Sergio is playing virtually every match in the Europa League and he deserves this continuity
However, Mourinho insists the decision to play Romero for tonight's trip to Belgium was purely tactical.
Mourinho said: "Sergio is playing virtually every match in the Europa League and he deserves this continuity.
"The fact he played against Sunderland is good for him.
Jose Mourinho's Manchester United drew 1-1 with Anderlecht Manchester United fans rage at starting XI to face Anderlecht Thu, April 13, 2017
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Manchester United fans are not happy with Jose Mourinho's team selection ahead of their Europa League quarter-final first leg against Anderlecht (8.05pm)
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Manchester United fans fume at the Red Devils' starting XI against Anderlecht
"We have two very good goalkeepers and we have two young keepers behind them both so we are fine."
United play Anderlecht in the return leg on Thursday (8.05pm).