Mignolet has fallen down the pecking order at Anfield and he has not played since the end of January.
And, according to TMW, the Belgian will be sold this summer and Casillas brought in as his replacement.
The Spaniard, who recently made his landmark 1000th professional appearance, is out of contract with Porto at the end of the season and will be available on a free transfer.
The 36-year-old is keen to keep playing in Europe and he has previously been a target for Newcastle United.
Liverpool are now ready to move for him, though, and bring him in to be back-up to Lorius Karius.