"First half he had more chances than me but the goalkeeper saved me, Alisson made big saves!" Alexander-Arnold said.
Liverpool won 2-0 when the teams last met on the pitch in January and United fan Grennan was gracious in his FIFA 20 defeat.
"Liverpool deserved the win in real life and they deserved it on FIFA!" Grennan said.
Raheem Sterling (MCI) 5-4 Wilfried Zaha (CRY) | Report and highlights
Tom Grennan (MUN) 1-5 Trent Alexander-Arnold (LIV)
Reece James (CHE) 0-8 Andre Gomes (EVE) | Report and highlights
Moussa Sissoko (TOT) 1-2 Christian Atsu (NEW) | Report and highlights
The prize fund is being donated to the #PlayersTogether initiative, created by Premier League players as a collective way of generating funds for the National Health Service and distributing them where they are needed most.