The Sweden striker yesterday put pen to paper on a three-year Baggies deal and watched his new team-mates from the stands during last night's 3-1 pre-season win at Walsall.
"Markus works hard for the team and his energy levels are fantastic," Clarke told the Albion website.
"He joins us after a seven-week break, which is not ideal, so his energy levels are not fantastic at the moment. But he trained for the first time with some of the players yesterday and he's excited about the challenge - and it is a challenge.
"Playing in the Premier League, no matter where you've been, it's a different league. And all the players we've brought in will take a little bit of time to adjust. Markus will take a bit of time physically to catch up with the rest.
"We'll work him as hard as we feel necessary between now and the start of the season. But, if he's not ready then, he'll be ready when he is."