The Croatian forward joined the Hammers on deadline day and has been praised by his former team-mate.
The Croatian centre forward agreed a deal to link-up with compatriot Slaven Bilic at the Boleyn Ground on deadline day, signing for the Hammers from Championship side Hull.
Jelavic has not had the best last 12 months, but showed in his time at both Rangers and Everton that he has the weaponry to score consistently.
Harper, who was part of the same Hull squad as the experienced attacker, has been singing the Eastern European forward’s praises following the conclusion of his move to West Ham.
“Jela is a battering ram of a centre forward,” the ex-Newcastle goalkeeper told BBC Sport.
“He might look ungainly but he is very effective. He is as fit as a fiddle, works his socks off and he can score goals.
"He has a reputation for one touch finishes which is fully justified. You're unlikely to see him beat three men and curl one into the top corner but has knack of scoring goals at important times.
“He will be a fantastic addition to the dressing room and his record proves when he is fit he has a decent goal return.”
West Ham showed their intent on transfer deadline day yesterday, adding four new faces to Bilic’s squad, with Alex Song, Victor Moses and Michail Antonio joining Jelavic at the London club.
The Croat will have a job on his hands to feature regularly for the Hammers, with Andy Carroll returning from injury and Diafra Sakho continuing to impress for the side.
Jelavic did not get the goals to keep Hull in the top flight last season, but the West Ham fans will hope that countryman Bilic can get the best out of him at the club.