Costello was a highly touted player entering the season but injuries limited him to just five games in 2019.
The quarterback should be eligible to play immediately in 2020 with one year left in college after spending four years with the program, seeing game action in three seasons.
Costello was at his best in 2018, totaling 3,540 passing yards and 29 touchdowns in 13 games while leading the Cardinal to a 9-4 record.
The 6'5" passer was considered a potential NFL draft prospect, even listed among the top 10 options at the position as recently as November by Bleacher Report's Matt Miller. His coach David Shaw also expected him to leave for the NFL after this season:
However, he was in and out of the lineup in 2019 and only played two games after September. He finished the year with just six touchdowns with three interceptions while Stanford struggled to 4-8 overall.
Despite the lost season, Costello will likely be a hot commodity in the transfer pool:
David Mills would presumably enter 2020 as Stanford's staring quarterback if Costello does indeed leave.