It was Escobar's choice to sign with someone where he could pitch out of the pen, because he believes it maybe less stress on his shoulder. I've heard differing arguments on protecting repaired arms, some say pitching once a week on a consistent rotation is less stress, while others say the shorter outings from out of the pen are better.
Throughout his career Escobar has been dominant in both roles: as a rookie in Toronto at the age of 21 y/o he had 14 saves with a 2.90 ERA and a 3-2 record. In 1999 as a 23 y/o, still with the Jays in his first full time exposure to the rotation he was a 14 games winner ( 5.69 ERA/ 176 innings). This is how versatile he is, in 2002 he had 38 saves with a 4.27 ERA and the very next year he was a 13 game winner in the rotation with a 4.29 ERA, both for Toronto in the tough AL East.
In 2004 he joined the Angels as their #2 starter, he won 11 games twice and 18 games in 2007, since leaving the AL east his ERA hasn't gone above 3.90.
At the time of the trade Harold Reynolds of MLB network, discussed in the linked video, the signing and speculated that the Mets may stretch him out into the rotation if he's healthy. Mack of Mack's Mets speculates about Kelvim's role and while he's speculating it's based on conversations he''s had.
The Mets SP2 After The All-Star Break:-This is just my opinion, which is based on some conversations I have had. It’s not etched in stone, but don’t be surprised if Kelvim Escobar starts off the season on target. If he does, and it will be as a reliever, you just may see him start a Joba-like conversion to starter, targeting himto become the SP2 by the all-star break.On paper, he has proven he is far better than anyone the Mets have, other than Johan Santana, and even is superior to Ben Sheets. source Mack's Mets