The Red Devils are yet to officially confirm the signing of the Shanghai Shenhua attacker, although it is being widely reported that the 30-year-old has agreed to join the club on loan for the rest of the season.
Stam has conceded that he did not see the move coming but has suggested that there will be little pressure on the Nigeria international to hit the ground running.
"It's a surprising signing, he's got nothing to lose. No-one was expecting this. He can just go in there and do his thing. Hopefully he'll do well as we all want to see Manchester United back up there," Stam told Sky Sports News.
Ighalo will not be involved in United's Premier League clash with Wolverhampton Wanderers on Saturday but could feature against Chelsea at Stamford Bridge on February 17.
Tottenham Hotspur and Inter Milan are also believed to have shown an interest in the latter stages of the transfer deadline.