United made a surprise move for Ighalo on January deadline day following a frantic search for attacking reinforcements, initially signing the Nigerian on loan until the end of the campaign.
Ighalo has scored four goals in his eight appearances for the Red Devils so far, leading to speculation that his boyhood club could look to make his "dream" move permanent.
Shenhua are still keen to keep hold of the 30-year-old and have reportedly offered him a lucrative £400,000-a-week contract to stay, but The Independent claims that they have lined up 35-year-old Martins as a potential replacement.
Martins has been without a club since leaving Shenhua in 2018 at the end of a two-year stay but could now be set for a return to action as cover for his compatriot.
The former Newcastle United, Inter Milan and Birmingham City striker scored 32 goals in just 59 games for Shanghai during his first spell at the club.