The midfielder has had an inconsistent spell so far.
But Mertesacker says: "It's really important to see him going on loan for a great experience.
“The Bundesliga sits right below the Premier League in terms of the tempo.
“Going from the under-23s to Germany then coming back to the Premier League a good step.
“Sometimes you get stuck in the 23s training bubble. You train wth the first team which looks nice but you need the competition."