The Reds' no.7 has been selected in Ten Hag's starting line-up for today's Premier League game away to Brentford, after coming on as a second-half substitute in the opening weekend defeat to Brighton & Hove Albion.
"It was short [pre-season] but Ronaldo is Ronaldo. He can do that," the United boss told our reporter Stewart Gardner.
"He proved that so often in the past. He's a really fit guy and he has to show that today."
Expanding further on the reasons for recalling Ronaldo, Erik told Sky Sports: "We had problems with numbers in the squad, in the offensive part.