Argentinian defender Otamendi is valued at £25million. City scout Rob Newman watched the 25-year-old in Porto’s 3-1 win against Sporting Lisbon last Sunday.
Otamendi is also being watched by Chelsea and Manchester United.
French youngster Mangala is rated at £40million and is being strongly linked with the likes of United, City and Chelsea.
Porto are keen on signing United midfielder Anderson on a loan deal in January.
The Daily Star reports that Porto are interested in any deal involving Anderson.