Ratings agency Morningstar has placed five of Pioneer's European fixed income funds under review following the news two key employees had been suspended after allegedly planning to launch a rival asset manager.
Morningstar said in a note that it would "continue to closely monitor" the potential impact of the suspension of Tanguy Le Saout, head of European fixed income, and portfolio construction unit boss Ali Chabaane.
Five funds have been affected - two directly managed by Mr Le Saout - and three others for which he led the team.
Earlier this week, Pioneer announced two employees had been suspended following an internal investigation concluding they had "acted against the commercial best interests of the firm by seeking to launch a competing asset management company".
Morningstar added: "While we appreciate that Pioneer has taken the issue seriously and acted swiftly to protect investors, we think the suspension is a serious event that warrants a reassessment of the fund's investment merit.
"In addition to his leadership role, Le Saout was one of the architects of the investment process applied here and was himself responsible for several key strategies."
However, Morningstar said the independence of other bond fund strategies meant it did not need to review all portfolios. Similarly, risk control processes remained in place after Mr Chabaane's departure, so equity funds need not be reviewed.
"Without minimising the importance of the event, Le Saout's suspension doesn't necessarily imply huge changes to the overall funds' active bets, at least in the short term," the note said.
Following the suspension, Mr Marasciulo has assumed leadership of the European fixed income team. Declan Murray, global chief of staff for investments, takes over Mr Chabaane's duties.
Funds under review by Morningstar:
Pioneer Euro Aggregate Bond
Pioneer Euro Bond
Pioneer Euro Corporate Bond
Pioneer Global Aggregate Bond
Pioneer Euro Distribution