Erik ten Hag rues absence of Eriksen and Casemiro in Manchester United midfield
Danish midfielder Eriksen was ruled out for up to three months in late January due to an ankle injury, while Casemiro is serving a four-match suspension after receiving his second red card of the season.
The Brazilian has been one of Ten Hag's most influential players since he was signed in a deal worth 70 million euros, with United losing four of their eight league games without him compared with three of 19 when he has played.
"They're two quality players in the midfield department, games will always be decided in midfield," Ten Hag told reporters ahead of Wednesday's home game against seventh-placed Brentford.
"So when you miss two quality players, it's clear. But you have a squad, if you don't have them you still have to win."
While Casemiro is suspended for two more matches - against Brentford and Everton - Ten Hag said Eriksen is back in training but not yet ready for action.
"Not for tomorrow but he's returning on the pitch. He's returning into team training today, so we have to see now how quickly he can make progress," Ten Hag added.
United's 2-0 loss to Newcastle United over the weekend dropped Ten Hag's side to fifth in the standings on goal difference.
Although United have progressed in the FA Cup and Europa League, they have not won in the league since mid-February.
That winless run includes a 7-0 thrashing by Liverpool and a goalless draw with bottom side Southampton.
Ten Hag was not downcast though.
"We played 60 minutes against Southampton with 10 men, we beat Fulham (in the FA Cup) - a Premier League opponent," he said.
"We beat Newcastle in the Carabao Cup, another Premier League opponent. We know we can beat Premier League teams.
"We had a very bad performance (against Newcastle on Sunday) because the opponent wanted it more than us. They were sharper."
