Nearly all players play injuries down because they want to continue playing.
Itís up to the management and medical staff to take that into account and make decisions for the good of the player and the team.
And indeed, this was an extremely important game, particularly with the trickier matches coming up.
Yeah they should make decisions but, when did it happen? If there were 20 minutes to go, then yeah, take him off as a precaution if nothing else. But if it's early in the first half (I really don't know when it happened) and Gylfi says it's ok, presumably they gave him until half time and he said he was fine to carry on. Which, again, presumably he was because people have said he played well.
The medical staff can't just take every player off when they feel a twinge, especially when they then go on to play the full 90.