After England struggled to 216 all out, Pakistan romped to a convincing, six-wicket win in Abu Dhabi in what proved a rousing farewell to Younis Khan, who was playing his last ODI.
Lanky paceman Mohammad Irfan rocked England with a wicket off the second ball of the game before finishing with 3-35 in his 10 overs.
1st ODI: Ton-up Hafeez guides Pakistan's victory over England
Morgan admitted the batting had let England down after the captain (76) and James Taylor (60) lifted the tourists from 14-3 to 147 before they lost four wickets in the space of 32 balls.
"We were looking at a minimum of 250 but getting there obviously wasn't easy on this type of wicket. We have to reassess that quickly," said Morgan.
"Probably after 20 overs, the ball started to hold up and grip more than it did in the previous few overs."
Younus Khan retires from one-day international cricket
Pakistan also wobbled at 41-3 before opener Mohammad Hafeez hit a brilliant, match-winning 102 not out to help his team overhaul the target in 43.4 overs.
"I think probably the big positive was having Pakistan at the same position at the halfway mark, and credit to the bowlers to get us in that position," said Morgan, praising Topley who grabbed 3-26 in his second ODI.
"He is a big asset for long-term. He is certainly somebody that we are looking at to nurture, particularly with his white-ball skills and he is very good for a 21-year-old," said Morgan.
Waqar Younis says England series 'important for ODI future'
Hafeez said Pakistan wanted to win the match for veteran batsman Younis, who retired from one-day cricket after the game.
"We were pumped up for Younis and although it was a shock for us to know that he was retiring but we want to give him the gift of a win and everyone chipped in," said Hafeez.
Younis finished his ODI career with 7,249 runs in 265 matches.
UAE will be colored Green this time around
Hafeez, who also hit 151 in the third Test in Sharjah as Pakistan clinched a 2-0 Test series win, said he was happy to stay in the runs.
"I am happy that I am contributing in Pakistan's wins and everything is going well for me," said Hafeez.
The remaining matches will be played in Sharjah (November 17) and Dubai (November 20), ahead of a three-match Twenty20 series.
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