[3] Vrousai entered the court of Karaiskakis Stadium in the 78th minute of the game, as the third substitute of Olympiacos' interim coach Christos Kontis, taking the place of Belgian international Kevin Mirallas.
Olympiacos got a 4–0 victory versus the Swiss team at Karaiskakis Stadium for the first leg of the round and Vrousai played as a late substitute for the last 12 minutes of the match.
On 19 December 2019, he scored an amazing long shot goal, the first of Olympiacos in the 1–2 road victory versus Aris Avato at Xanthi Ground for the last matchday of the Group Stage of the 2018–19 Greek Football Cup.
If Willem II wish to sign Vrousai on a permanent basis, the release clause amounts to €1 million, while in the event Olympiacos wanted to end his loan prematurely this summer, they would have to pay the Eredivisie team a fee.
[4] On 20 January 2019, Marios Vrousai scored in his debut with Willem's jersey, sealing a 2–0 home win game against Breda after Vangelis Pavlidis assist.