There were some doubts about whether the veteran Swede could handle the pace of the Premier League following his switch from Paris Saint-German during the off-season.
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But Ibrahimovic has silenced his critics with a fine debut campaign at Old Trafford thus far, scoring 15 goals in 24 top-flight appearances.
The 35-year-old striker took his tally to 24 for the Red Devils when he came off the bench to grab the winner as they beat Blackburn 2-1 in the FA Cup on Sunday, and Mourinho says he was convinced the talisman would deliver for his new club.
“I don’t know the number of goals [Ibrahimovic has scored], I don’t know,” the Portuguese coach told a media conference on Monday.
“I knew that he would be a very important player for us. I knew that for sure. One more goal, one less goal, but I knew that the contribution would be massive for us.”